Fix Volume slider is captured in screenshot done in touchview mode

ash/common/system/audio/tray_audio.h

Index: ash/common/system/audio/tray_audio.h
diff --git a/ash/common/system/audio/tray_audio.h b/ash/common/system/audio/tray_audio.h
index 00baf8ccfb94b8ce2a7d072b34d9d3700fa56810..2fc1c58cc777c3d05748190a398e0630d9a9593d 100644
--- a/ash/common/system/audio/tray_audio.h
+++ b/ash/common/system/audio/tray_audio.h
@@ -11,6 +11,7 @@
 
 #include "ash/common/system/audio/audio_observer.h"
 #include "ash/common/system/tray/tray_image_item.h"
+#include "ash/common/wm/maximize_mode/maximize_mode_screenshot_observer.h"
 #include "base/macros.h"
 #include "ui/display/display_observer.h"
 
@@ -26,7 +27,8 @@ class VolumeView;
 
 class TrayAudio : public TrayImageItem,
                   public AudioObserver,
-                  public display::DisplayObserver {
+                  public display::DisplayObserver,
+                  public MaximizeModeScreenshotObserver {
  public:
   TrayAudio(SystemTray* system_tray,
             std::unique_ptr<system::TrayAudioDelegate> audio_delegate);
@@ -69,6 +71,9 @@ class TrayAudio : public TrayImageItem,
   void OnActiveOutputNodeChanged() override;
   void OnActiveInputNodeChanged() override;
 
+  // Overridden from MaximizeModeScreenshotObserver.
+  void OnWillTakeScreenshotInMaximizeMode() override;
+
   void ChangeInternalSpeakerChannelMode();
 
   DISALLOW_COPY_AND_ASSIGN(TrayAudio);