NOT FOR LANDING Introduce and use WebMediaSession

public/platform/WebMediaSession.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef InternalsWebAudio_h
-#define InternalsWebAudio_h
+#ifndef WebMediaSession_h
+#define WebMediaSession_h
 
 namespace blink {
 
-class Internals;
+class WebMediaSession {
+public:
+    virtual ~WebMediaSession() { }
 
-class InternalsWebAudio {
-public:
-    static unsigned audioHandlerCount(Internals&);
+    virtual void activate() = 0;
+    virtual void deactivate() = 0;
 };
 
 } // namespace blink
 
-#endif // InternalsWebAudio_h
+#endif // WebMediaSession_h