Make Encoded*Chunk interfaces immutable.

Provides a readInto() method for copying the data out.

Fixes #80. Also, see #127.

index.src.html

@@ -1720,7 +1720,9 @@
   constructor(EncodedAudioChunkInit init);
   readonly attribute EncodedAudioChunkType type;
   readonly attribute unsigned long long timestamp;  // microseconds
-  readonly attribute ArrayBuffer data;
+  readonly attribute unsigned long byteLength;
+
+  undefined readInto(ArrayBufferView dst);
 };
 
 dictionary EncodedAudioChunkInit {
@@ -1735,27 +1737,39 @@
 };
 </xmp>
 
+### Internal Slots ### {#encodedaudiochunk-internal-slots}
+: <dfn attribute for=EncodedAudioChunk>[[internal data]]</dfn></dt>
+:: An array of bytes repsenting the encoded chunk data.
+
 ### Constructors ###{#encodedaudiochunk-constructors}
 <dfn constructor for=EncodedAudioChunk title="EncodedAudioChunk(init)">
   EncodedAudioChunk(init)
 </dfn>
 1. Let |chunk| be a new {{EncodedAudioChunk}} object, initialized as follows
-    1. Assign `init.type` to `chunk.type`.
-    2. Assign `init.timestamp` to `chunk.timestamp`.
-    3. Assign a copy of `init.data` to `chunk.data`.
+    1. Assign `init.type` to {{EncodedAudioChunk/type}}.
+    2. Assign `init.timestamp` to {{EncodedAudioChunk/timestamp}}.
+    3. Assign a copy of `init.data` to {{EncodedAudioChunk/[[internal data]]}}.
+    4. Assign `init.data.byteLength` to {{EncodedAudioChunk/byteLength}};
 5. Return |chunk|.
 
 ### Attributes ###{#encodedaudiochunk-attributes}
 <dl>
-  <dt><dfn attribute for=EncodedAudioChunk>type</dfn></dt>
-  <dd>Describes whether the chunk is a key frame.</dd>
+: <dfn attribute for=EncodedAudioChunk>type</dfn>
+:: Describes whether the chunk is a key frame.
 
-  <dt><dfn attribute for=EncodedAudioChunk>timestamp</dfn></dt>
-  <dd>The presentation timestamp, given in microseconds.</dd>
+: <dfn attribute for=EncodedAudioChunk>timestamp</dfn>
+:: The presentation timestamp, given in microseconds.
 
-  <dt><dfn attribute for=EncodedAudioChunk>data</dfn></dt>
-  <dd>A sequence of bytes containing encoded audio data.</dd>
-</dl>
+: <dfn attribute for=EncodedAudioChunk>byteLength</dfn>
+:: The byte length of {{EncodedAudioChunk/[[internal data]]}}.
+
+### Methods ###{#encodedaudiochunk-methods}
+: <dfn method for=EncodedAudioChunk>readInto(dst)</dfn>
+:: When invoked, run these steps:
+    1. If {{EncodedAudioChunk/byteLength}} is greater than
+        |dst|.`byteLength`,
+        throw a {{TypeError}}.
+    2. Copy the {{EncodedAudioChunk/[[internal data]]}} into |dst|.
 
 EncodedVideoChunk Interface{#encodedvideochunk-interface}
 -----------------------------------------------------------
@@ -1766,7 +1780,9 @@
   readonly attribute EncodedVideoChunkType type;
   readonly attribute unsigned long long timestamp;  // microseconds
   readonly attribute unsigned long long? duration;  // microseconds
-  readonly attribute ArrayBuffer data;
+  readonly attribute unsigned long byteLength;
+
+  undefined readInto(ArrayBufferView dst);
 };
 
 dictionary EncodedVideoChunkInit {
@@ -1782,33 +1798,44 @@
 };
 </xmp>
 
+### Internal Slots ### {#encodedvideochunk-internal-slots}
+: <dfn attribute for=EncodedVideoChunk>[[internal data]]</dfn></dt>
+:: An array of bytes repsenting the encoded chunk data.
+
 ### Constructors ###{#encodedvideochunk-constructors}
 <dfn constructor for=EncodedVideoChunk title="EncodedVideoChunk(init)">
   EncodedVideoChunk(init)
 </dfn>
 1. Let |chunk| be a new {{EncodedVideoChunk}} object, initialized as follows
-    1. Assign `init.type` to `chunk.type`.
-    2. Assign `init.timestamp` to `chunk.timestamp`.
+    1. Assign `init.type` to {{EncodedVideoChunk/type}}.
+    2. Assign `init.timestamp` to {{EncodedVideoChunk/timestamp}}.
     3. If duration is present in init, assign `init.duration` to
-        `chunk.duration`. Otherwise, assign null to `chunk.duration`.
-2. Assign a copy of `init.data` to `chunk.data`.
+        {{EncodedVideoChunk/duration}}. Otherwise, assign `null` to
+        {{EncodedVideoChunk/duration}}.
+    4. Assign a copy of `init.data` to {{EncodedVideoChunk/[[internal data]]}}.
+    5. Assign `init.data.byteLength` to {{EncodedVideoChunk/byteLength}};
 3. Return |chunk|.
 
 ### Attributes ###{#encodedvideochunk-attributes}
-<dl>
-  <dt><dfn attribute for=EncodedVideoChunk>type</dfn></dt>
-  <dd>Describes whether the chunk is a key frame or not.</dd>
+: <dfn attribute for=EncodedVideoChunk>type</dfn>
+:: Describes whether the chunk is a key frame or not.
 
-  <dt><dfn attribute for=EncodedVideoChunk>timestamp</dfn></dt>
-  <dd>The presentation timestamp, given in microseconds.</dd>
+: <dfn attribute for=EncodedVideoChunk>timestamp</dfn>
+:: The presentation timestamp, given in microseconds.
 
-  <dt><dfn attribute for=EncodedVideoChunk>duration</dfn></dt>
-  <dd>The presentation duration, given in microseconds.</dd>
+: <dfn attribute for=EncodedVideoChunk>duration</dfn>
+:: The presentation duration, given in microseconds.
 
-  <dt><dfn attribute for=EncodedVideoChunk>data</dfn></dt>
-  <dd>A sequence of bytes containing encoded video data.</dd>
-</dl>
+: <dfn attribute for=EncodedVideoChunk>byteLength</dfn>
+:: The byte length of {{EncodedVideoChunk/[[internal data]]}}.
 
+### Methods ###{#encodedvideochunk-methods}
+: <dfn method for=EncodedVideoChunk>readInto(dst)</dfn>
+:: When invoked, run these steps:
+    1. If {{EncodedVideoChunk/byteLength}} is greater than
+        |dst|.`byteLength`,
+        throw a {{TypeError}}.
+    2. Copy the {{EncodedVideoChunk/[[internal data]]}} into |dst|.
 
 Raw Media Interfaces (Frames){#raw-media-interfaces}
 ====================================================
@@ -2300,9 +2327,6 @@
 This concern is mitigated by ensuring that input and output interfaces are
 immutable.
 
-ISSUE: EncodedVideoChunk and EncodedAudioChunk currently expose a mutable
-data. See <a href="https://github.com/w3c/webcodecs/issues/80">#80</a>.
-
 Privacy Considerations{#privacy-considerations}
 ===============================================
 The primary privacy impact is an increased ability to fingerprint users by