Rewrite bitrate test

Use XMLHttpRequest to measure the actual loading stage and get
the actual number of bytes loaded.

src/apiClient.js

@@ -9,6 +9,8 @@ const reportRateLimits = {
 
 /** Approximate LAN bitrate */
 const LAN_BITRATE = 140000000;
+/** Bitrate test timeout in milliseconds */
+const BITRATETEST_TIMEOUT = 5000;
 
 function redetectBitrate(instance) {
     stopBitrateDetection(instance);
@@ -746,22 +748,66 @@ class ApiClient {
     }
 
     getDownloadSpeed(byteSize) {
-        const url = this.getUrl('Playback/BitrateTest', {
-            Size: byteSize
-        });
+        return new Promise((resolve, reject) => {
+            const url = this.getUrl('Playback/BitrateTest', {
+                Size: byteSize
+            });
 
-        const now = new Date().getTime();
+            console.log(`Requesting ${url}`);
 
-        return this.ajax({
-            type: 'GET',
-            url,
-            timeout: 5000
-        }).then(() => {
-            const responseTimeSeconds = (new Date().getTime() - now) / 1000;
-            const bytesPerSecond = byteSize / responseTimeSeconds;
-            const bitrate = Math.round(bytesPerSecond * 8);
+            const xhr = new XMLHttpRequest;
+
+            xhr.open('GET', url, true);
+
+            const headers = {
+                'Cache-Control': 'no-cache'
+            };
+
+            this.setRequestHeaders(headers);
+
+            for (const key in headers) {
+                xhr.setRequestHeader(key, headers[key]);
+            }
+
+            const timeout = setTimeout(() => {
+                xhr.abort();
+                reject('BitrateTest timeout');
+            }, BITRATETEST_TIMEOUT);
+
+            let startTime;
+
+            xhr.onreadystatechange = () => {
+                if (xhr.readyState == XMLHttpRequest.HEADERS_RECEIVED) {
+                    startTime = performance.now();
+                }
+            };
+
+            xhr.onload = (e) => {
+                clearTimeout(timeout);
+
+                if (xhr.status < 400) {
+                    const responseTimeSeconds = (performance.now() - startTime) * 1e-3;
+
+                    // Some older web engines (WebKit in webOS 1.2) may not have `ProgressEvent.loaded` field.
+                    const loaded = e.loaded || byteSize;
+
+                    const bytesPerSecond = loaded / responseTimeSeconds;
+                    const bitrate = Math.round(bytesPerSecond * 8);
+
+                    console.debug(`BitrateTest ${loaded} bytes loaded (${byteSize} requested) in ${responseTimeSeconds} seconds -> ${bitrate} bps`);
+
+                    resolve(bitrate);
+                } else {
+                    reject(`BitrateTest failed with ${xhr.status} status`);
+                }
+            };
+
+            xhr.onerror = () => {
+                clearTimeout(timeout);
+                reject('BitrateTest error');
+            };
 
-            return bitrate;
+            xhr.send(null);
         });
     }