Enable the volume slider in Ash for windows.

ash/system/win/audio/tray_audio_win.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_WIN_AUDIO_TRAY_AUDIO_WIN_H_
+#define ASH_SYSTEM_WIN_AUDIO_TRAY_AUDIO_WIN_H_
+
+#include "ash/ash_export.h"
+#include "ash/system/audio/tray_audio.h"
+#include "base/memory/scoped_ptr.h"
+
+namespace ash {
+
+namespace internal {
+
+class TrayAudioWin : public TrayAudio {
+ public:
+  explicit TrayAudioWin(SystemTray* system_tray);
+  virtual ~TrayAudioWin();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(TrayAudioWin);
+};
+
+}  // namespace internal
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_WIN_AUDIO_TRAY_AUDIO_WIN_H_