Fixed the audio backgrounding bug

content/browser/renderer_host/media/audio_renderer_host.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.
 //
 // AudioRendererHost serves audio related requests from AudioRenderer which
 // lives inside the render process and provide access to audio hardware.
 //
 // This class is owned by BrowserRenderProcessHost, and instantiated on UI
 // thread, but all other operations and method calls happen on IO thread, so we
 // need to be extra careful about the lifetime of this object. AudioManager is a
@@ skipping 198 matching lines @@
   media::AudioManager* const audio_manager_;
   AudioMirroringManager* const mirroring_manager_;
   scoped_ptr<media::AudioLog> audio_log_;
 
   // Used to access to AudioInputDeviceManager.
   MediaStreamManager* media_stream_manager_;
 
   // A map of stream IDs to audio sources.
   AudioEntryMap audio_entries_;
 
-  // The number of streams in the playing state.
+  // The number of streams in the playing state. Atomic read safe from any
+  // thread, but should only be updated from the IO thread.
   base::AtomicRefCount num_playing_streams_;
 
   // Salt required to translate renderer device IDs to raw device IDs
   ResourceContext::SaltCallback salt_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioRendererHost);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_MEDIA_AUDIO_RENDERER_HOST_H_