Move SbPlayerPipeline declaration to .h file (youtube#535)

The declaration of SbPlayerPipeline was in sbplayer_pipeline.cc so other
code won't depend on SbPlayerPipeline directly.  Instead, they'd create
it using Pipeline::Create() (which creates an SbPlayerPipeline
internally) and access it via the Pipeline interface.

We are trying to reduce the size of sbplayer_pipeline.cc (it's >1700
loc as of today).  Moving the declaration to an .h file reduces ~400
lines, which outweighs the benefit mentioned above.

There are no functional changes, except that Pipeline::Create() is
removed, as now SbPlayerPipeline can be created directly.

b/285639635

cobalt/media/BUILD.gn

@@ -48,6 +48,7 @@ component("media") {
     "base/sbplayer_interface.cc",
     "base/sbplayer_interface.h",
     "base/sbplayer_pipeline.cc",
+    "base/sbplayer_pipeline.h",
     "base/sbplayer_set_bounds_helper.cc",
     "base/sbplayer_set_bounds_helper.h",
     "decoder_buffer_allocator.cc",

cobalt/media/base/pipeline.h

@@ -92,17 +92,6 @@ class MEDIA_EXPORT Pipeline : public base::RefCountedThreadSafe<Pipeline> {
       OnEncryptedMediaInitDataEncounteredCB;
 #endif  // SB_HAS(PLAYER_WITH_URL)
 
-  static scoped_refptr<Pipeline> Create(
-      SbPlayerInterface* interface, PipelineWindow window,
-      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
-      const GetDecodeTargetGraphicsContextProviderFunc&
-          get_decode_target_graphics_context_provider_func,
-      bool allow_resume_after_suspend, bool allow_batched_sample_write,
-#if SB_API_VERSION >= 15
-      SbTime audio_write_duration_local, SbTime audio_write_duration_remote,
-#endif  // SB_API_VERSION >= 15
-      MediaLog* media_log, DecodeTargetProvider* decode_target_provider);
-
   virtual ~Pipeline() {}
 
   virtual void Suspend() {}