Revert of Pass task runners to AudioManager constructor.

media/audio/mock_audio_manager.h

Index: media/audio/mock_audio_manager.h
diff --git a/media/audio/mock_audio_manager.h b/media/audio/mock_audio_manager.h
index 33f7e5eace700d5d99c849bb6b6f033493cdefc9..60a8b1ea99e67b147e8d5f5fd2c83c0bfc1d474d 100644
--- a/media/audio/mock_audio_manager.h
+++ b/media/audio/mock_audio_manager.h
@@ -11,13 +11,19 @@
 namespace media {
 
 // This class is a simple mock around AudioManager, used exclusively for tests,
-// which avoids to use the actual (system and platform dependent) AudioManager.
-// Some bots does not have input devices, thus using the actual AudioManager
-// would causing failures on classes which expect that.
+// which has the following purposes:
+// 1) Avoids to use the actual (system and platform dependent) AudioManager.
+//    Some bots does not have input devices, thus using the actual AudioManager
+//    would causing failures on classes which expect that.
+// 2) Allows the mock audio events to be dispatched on an arbitrary thread,
+//    rather than forcing them on the audio thread, easing their handling in
+//    browser tests (Note: sharing a thread can cause deadlocks on production
+//    classes if WaitableEvents or any other form of lock is used for
+//    synchronization purposes).
 class MockAudioManager : public media::AudioManager {
  public:
   explicit MockAudioManager(
-      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner);
 
   bool HasAudioOutputDevices() override;
 
@@ -44,6 +50,9 @@
       const media::AudioParameters& params,
       const std::string& device_id) override;
 
+  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() override;
+  scoped_refptr<base::SingleThreadTaskRunner> GetWorkerTaskRunner() override;
+
   void AddOutputDeviceChangeListener(AudioDeviceListener* listener) override;
   void RemoveOutputDeviceChangeListener(AudioDeviceListener* listener) override;
 
@@ -62,6 +71,8 @@
   ~MockAudioManager() override;
 
  private:
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(MockAudioManager);
 };