Fix the volume controlling behaviors.

chrome/browser/chromeos/audio/audio_handler.h

Index: chrome/browser/chromeos/audio/audio_handler.h
diff --git a/chrome/browser/chromeos/audio/audio_handler.h b/chrome/browser/chromeos/audio/audio_handler.h
index 27027f4d949d3a6ad8f00f3062eeaf1ccd4cc75f..c115b8784249cda2e025641da792641795fdbb21 100644
--- a/chrome/browser/chromeos/audio/audio_handler.h
+++ b/chrome/browser/chromeos/audio/audio_handler.h
@@ -33,6 +33,10 @@ class AudioHandler {
   static void Initialize();
   static void Shutdown();
 
+  // Same as Initialize but using the specified audio mixer.  It takes
+  // ownership of |mixer|.
+  static void InitializeForTesting(AudioMixer* mixer);
+
   // GetInstance returns NULL if not initialized or if already shutdown.
   static AudioHandler* GetInstance();
 
@@ -62,7 +66,8 @@ class AudioHandler {
   // and constructor/destructor private as recommended for Singletons.
   friend struct DefaultSingletonTraits<AudioHandler>;
 
-  AudioHandler();
+  // Takes ownership of |mixer|.
+  explicit AudioHandler(AudioMixer* mixer);
   virtual ~AudioHandler();
 
   scoped_ptr<AudioMixer> mixer_;