Consolidate volume control code.

ash/volume_control_delegate.h

Index: ash/volume_control_delegate.h
diff --git a/ash/volume_control_delegate.h b/ash/volume_control_delegate.h
index 24410f17e6d3628a12a702c6dcb13759dc7e0ba1..db2a99bc85ecd487d166146cfefd9cae0b7f84ce 100644
--- a/ash/volume_control_delegate.h
+++ b/ash/volume_control_delegate.h
@@ -20,6 +20,18 @@ class VolumeControlDelegate {
   virtual bool HandleVolumeDown(const ui::Accelerator& accelerator) = 0;
   virtual bool HandleVolumeUp(const ui::Accelerator& accelerator) = 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. The range is [0, 1.0].
+  virtual float GetVolumeLevel() const = 0;
+
+  // Sets the volume level. The range is [0, 1.0].
+  virtual void SetVolumeLevel(float level) = 0;
+
   // Requests that the volume be set to |percent|, in the range
   // [0.0, 100.0].
   virtual void SetVolumePercent(double percent) = 0;