Fix MSE GC, make it less aggressive, more spec-compliant (Blink CL)

Source/modules/mediasource/MediaSource.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 83 matching lines @@
     bool hasPendingActivity() const override;
     void stop() override;
 
     // URLRegistrable interface
     URLRegistry& registry() const override;
 
     // Used by SourceBuffer.
     void openIfInEndedState();
     bool isOpen() const;
     void setSourceBufferActive(SourceBuffer*);
+    HTMLMediaElement* mediaElement() const;
 
     // Used by MediaSourceRegistry.
     void addedToRegistry();
     void removedFromRegistry();
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit MediaSource(ExecutionContext*);
 
@@ skipping 17 matching lines @@
 
     Member<SourceBufferList> m_sourceBuffers;
     Member<SourceBufferList> m_activeSourceBuffers;
 
     bool m_isAddedToRegistry;
 };
 
 } // namespace blink
 
 #endif // MediaSource_h