Revert of Pass task runners to AudioManager constructor.

media/audio/mock_audio_manager.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 MEDIA_AUDIO_MOCK_AUDIO_MANAGER_H_
 #define MEDIA_AUDIO_MOCK_AUDIO_MANAGER_H_
 
 #include "base/macros.h"
 #include "media/audio/audio_manager.h"
 
 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;
 
   bool HasAudioInputDevices() override;
 
   base::string16 GetAudioInputDeviceModel() override;
 
   void ShowAudioInputSettings() override;
 
   void GetAudioInputDeviceNames(media::AudioDeviceNames* device_names) override;
 
   void GetAudioOutputDeviceNames(
       media::AudioDeviceNames* device_names) override;
 
   media::AudioOutputStream* MakeAudioOutputStream(
       const media::AudioParameters& params,
       const std::string& device_id) override;
 
   media::AudioOutputStream* MakeAudioOutputStreamProxy(
       const media::AudioParameters& params,
       const std::string& device_id) override;
 
   media::AudioInputStream* MakeAudioInputStream(
       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;
 
   AudioParameters GetDefaultOutputStreamParameters() override;
   AudioParameters GetOutputStreamParameters(
       const std::string& device_id) override;
   AudioParameters GetInputStreamParameters(
       const std::string& device_id) override;
   std::string GetAssociatedOutputDeviceID(
       const std::string& input_device_id) override;
 
   scoped_ptr<AudioLog> CreateAudioLog(
       AudioLogFactory::AudioComponent component) override;
 
  protected:
   ~MockAudioManager() override;
 
  private:
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(MockAudioManager);
 };
 
 }  // namespace media.
 
 #endif  // MEDIA_AUDIO_MOCK_AUDIO_MANAGER_H_