Add metrics regarding concurrent audible tabs in Chromium.

content/browser/media/media_web_contents_observer.h

 // Copyright 2014 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_MEDIA_WEB_CONTENTS_OBSERVER_H_
 #define CONTENT_BROWSER_MEDIA_MEDIA_WEB_CONTENTS_OBSERVER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 10 matching lines @@
 
 // This class manages all RenderFrame based media related managers at the
 // browser side. It receives IPC messages from media RenderFrameObservers and
 // forwards them to the corresponding managers. The managers are responsible
 // for sending IPCs back to the RenderFrameObservers at the render side.
 class CONTENT_EXPORT MediaWebContentsObserver : public WebContentsObserver {
  public:
   explicit MediaWebContentsObserver(WebContents* web_contents);
   ~MediaWebContentsObserver() override;
 
-  // Called when the audible state has changed.  If inaudible any audio power
-  // save blockers are released.
-  void MaybeUpdateAudibleState(bool recently_audible);
+  // Called by WebContentsImpl when the audible state may have changed.
+  void MaybeUpdateAudibleState();
 
   // WebContentsObserver implementation.
+  void WebContentsDestroyed() override;
   void RenderFrameDeleted(RenderFrameHost* render_frame_host) override;
   bool OnMessageReceived(const IPC::Message& message,
                          RenderFrameHost* render_frame_host) override;
   void WasShown() override;
   void WasHidden() override;
 
   bool has_audio_power_save_blocker_for_testing() const {
     return audio_power_save_blocker_;
   }
 
@@ skipping 41 matching lines @@
   ActiveMediaPlayerMap active_video_players_;
   scoped_ptr<PowerSaveBlocker> audio_power_save_blocker_;
   scoped_ptr<PowerSaveBlocker> video_power_save_blocker_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaWebContentsObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_MEDIA_MEDIA_WEB_CONTENTS_OBSERVER_H_