Add a method to tab_utils.h to find out whether a tab is playing audio.

content/public/browser/media_observer.h

 // Copyright (c) 2012 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_PUBLIC_BROWSER_MEDIA_OBSERVER_H_
 #define CONTENT_PUBLIC_BROWSER_MEDIA_OBSERVER_H_
 
 #include "content/public/browser/media_request_state.h"
 #include "content/public/common/media_stream_request.h"
 
@@ skipping 23 matching lines @@
   virtual void OnVideoCaptureDevicesChanged(
       const MediaStreamDevices& devices) = 0;
 
   // Called when a media request changes state.
   virtual void OnMediaRequestStateChanged(
       int render_process_id,
       int render_view_id,
       const MediaStreamDevice& device,
       MediaRequestState state) = 0;
 
+  // Called when an audio stream is played or paused.
+  virtual void OnAudioStreamPlayingChanged(
+      int render_process_id,
+      int render_view_id,
+      int stream_id,
+      bool playing) = 0;
+
  protected:
   virtual ~MediaObserver() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_MEDIA_OBSERVER_H_