Work around for HDMI audio output rediscovering transistion loss.

ash/system/chromeos/audio/tray_audio_delegate_chromeos.h

 // Copyright 2014 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 ASH_SYSTEM_AUDIO_TRAY_AUDIO_DELEGATE_CHROMEOS_H_
 #define ASH_SYSTEM_AUDIO_TRAY_AUDIO_DELEGATE_CHROMEOS_H_
 
 #include "ash/ash_export.h"
 #include "ash/system/audio/tray_audio_delegate.h"
 #include "base/compiler_specific.h"
 
 namespace ash {
 namespace system {
 
 class ASH_EXPORT TrayAudioDelegateChromeOs : public TrayAudioDelegate {
  public:
   ~TrayAudioDelegateChromeOs() override {}
 
   // Overridden from TrayAudioDelegate.
   void AdjustOutputVolumeToAudibleLevel() override;
   int GetOutputDefaultVolumeMuteLevel() override;
   int GetOutputVolumeLevel() override;
   int GetActiveOutputDeviceIconId() override;
   bool HasAlternativeSources() override;
   bool IsOutputAudioMuted() override;
   void SetOutputAudioIsMuted(bool is_muted) override;
   void SetOutputVolumeLevel(int level) override;
   void SetInternalSpeakerChannelMode(AudioChannelMode mode) override;
+  void SetActiveHDMIOutoutRediscoveringIfNecessary(
+      bool force_rediscovering) override;
 };
 
 }  // namespace system
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_AUDIO_TRAY_AUDIO_DELEGATE_CHROMEOS_H_