Make audio volume icon HDC

ash/system/tray/system_tray_delegate.h

 // Copyright (c) 2012 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_TRAY_SYSTEM_TRAY_DELEGATE_H_
 #define ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 130 matching lines @@
 
   // Shows help.
   virtual void ShowHelp() = 0;
 
   // Is the system audio muted?
   virtual bool IsAudioMuted() const = 0;
 
   // Mutes/Unmutes the audio system.
   virtual void SetAudioMuted(bool muted) = 0;
 
-  // Gets the volume level.
+  // Gets the volume level. The range is [0, 1.0].
   virtual float GetVolumeLevel() const = 0;
 
-  // Sets the volume level.
+  // Sets the volume level. The range is [0, 1.0].
   virtual void SetVolumeLevel(float level) = 0;
 
   // Gets whether the caps lock is on.
   virtual bool IsCapsLockOn() const = 0;
 
   // Sets the caps lock status to |enabled|.
   virtual void SetCapsLockEnabled(bool enabled) = 0;
 
   // Attempts to shut down the system.
   virtual void ShutDown() = 0;
@@ skipping 111 matching lines @@
   // Opens the cellular network specific URL.
   virtual void ShowCellularURL(const std::string& url) = 0;
 
   // Shows UI for changing proxy settings.
   virtual void ChangeProxySettings() = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_