Work around for HDMI audio output rediscovering transistion loss.

ash/system/chromeos/audio/tray_audio_chromeos.h

Index: ash/system/chromeos/audio/tray_audio_chromeos.h
diff --git a/ash/system/chromeos/audio/tray_audio_chromeos.h b/ash/system/chromeos/audio/tray_audio_chromeos.h
index a20a49d7f51a672f9c6d8d0e415cb8c2e2f5d8f9..ed313e0e051d939e6c1c6c380eb438ba852e2c0a 100644
--- a/ash/system/chromeos/audio/tray_audio_chromeos.h
+++ b/ash/system/chromeos/audio/tray_audio_chromeos.h
@@ -8,13 +8,16 @@
 #include "ash/ash_export.h"
 #include "ash/system/audio/tray_audio.h"
 #include "base/memory/scoped_ptr.h"
+#include "chromeos/dbus/power_manager_client.h"
 
 namespace ash {
 namespace tray {
 class AudioDetailedView;
 }
 
-class ASH_EXPORT TrayAudioChromeOs : public TrayAudio {
+class ASH_EXPORT TrayAudioChromeOs
+    : public TrayAudio,
+      public chromeos::PowerManagerClient::Observer {
  public:
   explicit TrayAudioChromeOs(SystemTray* system_tray);
   ~TrayAudioChromeOs() override;
@@ -28,6 +31,15 @@ class ASH_EXPORT TrayAudioChromeOs : public TrayAudio {
   views::View* CreateDetailedView(user::LoginStatus status) override;
   void DestroyDetailedView() override;
 
+  // Overridden from gfx::DisplayObserver.
+  void OnDisplayAdded(const gfx::Display& new_display) override;
+  void OnDisplayRemoved(const gfx::Display& old_display) override;
+  void OnDisplayMetricsChanged(const gfx::Display& display,
+                               uint32_t changed_metrics) override;
+
+  // Overriden from chromeos::PowerManagerClient::Observer.
+  void SuspendDone(const base::TimeDelta& sleep_duration) override;
+
   tray::AudioDetailedView* audio_detail_view_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayAudioChromeOs);