Add about_flag for experimenting MediaSession on Desktop

content/browser/media/session/pepper_playback_observer.h

 // Copyright 2016 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 CONTENT_BROWSER_MEDIA_SESSION_PEPPER_PLAYBACK_OBSERVER_H_
 #define CONTENT_BROWSER_MEDIA_SESSION_PEPPER_PLAYBACK_OBSERVER_H_
 
 #include <stdint.h>
 #include <map>
 #include <memory>
 
 #include "base/macros.h"
 
 namespace content {
 
 class PepperPlayerDelegate;
 class WebContentsImpl;
 
 // Class observing Pepper playback changes from WebContents, and update
 // MediaSession accordingly. Can only be a member of WebContentsImpl and must be
 // destroyed in ~WebContentsImpl().
 class PepperPlaybackObserver {
  public:
   explicit PepperPlaybackObserver(WebContentsImpl* contents);
   virtual ~PepperPlaybackObserver();
 
   void PepperInstanceCreated(int32_t pp_instance);
   void PepperInstanceDeleted(int32_t pp_instance);
+  // This method is called when a Pepper instance starts making sound.
   void PepperStartsPlayback(int32_t pp_instance);
+  // This method is called when a Pepper instance stops making sound.
   void PepperStopsPlayback(int32_t pp_instance);
 
  private:
   // Owning PepperPlayerDelegates.
   using PlayersMap =
       std::map<int32_t, std::unique_ptr<PepperPlayerDelegate>>;
   PlayersMap players_map_;
 
   // Map for whether Pepper players have ever played sound.
   // Used for recording UMA.
   using PlayersPlayedSoundMap =
       std::map<int32_t, bool>;
   PlayersPlayedSoundMap players_played_sound_map_;
 
   // Weak reference to WebContents.
   WebContentsImpl* contents_;
 
   DISALLOW_COPY_AND_ASSIGN(PepperPlaybackObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_MEDIA_SESSION_PEPPER_PLAYBACK_OBSERVER_H_