Bug 855655 - Part 1: Add new attribute - volumeControlChannel into AudioChannelManager. sr=sicking

dom/webidl/AudioChannelManager.webidl

@@ -24,4 +24,10 @@ interface AudioChannelManager : EventTarget {
    */
   [SetterThrows]
   attribute EventHandler onheadphoneschange;
+
+  /**
+   * Indicates which audio channel is used to adjust volume when pressing HW
+   * volume keys.
+   */
+  attribute DOMString volumeControlChannel;
 };