[Refactor] Introduce MediaDelegate

ash/media_delegate.h

+// Copyright 2013 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_MEDIA_DELEGATE_H_
+#define ASH_MEDIA_DELEGATE_H_
+
+namespace ash {
+
+// A delegate class to control media playback.
+class MediaDelegate {
+ public:
+  MediaDelegate() {}
+  virtual ~MediaDelegate() {}

[stevenjb, 2013/10/31 20:40:03]

Protect destructor or use DISALLOW...
nit: constructor unnecessary

[oshima, 2013/11/01 00:30:25]

owned by shell, so can't be protected, and i believe
pur interface doesn't need DISALLOW_ (confirmed 
window_observer/delegate, shell_delegate and others don't have it)
Done.

+
+  // Handles the Next Track Media shortcut key.
+  virtual void HandleMediaNextTrack() = 0;
+
+  // Handles the Play/Pause Toggle Media shortcut key.
+  virtual void HandleMediaPlayPause() = 0;
+
+  // Handles the Previous Track Media shortcut key.
+  virtual void HandleMediaPrevTrack() = 0;
+};
+
+}  // namespace ash
+
+#endif  // ASH_MEDIA_DELEGATE_H_