Remove unnecessary attribute "gain" in AudioBuffer and AudioBufferSourceNode interfaces

Source/modules/webaudio/AudioBufferSourceNode.idl

 /*
  * Copyright (C) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 18 matching lines @@
 ] interface AudioBufferSourceNode : AudioSourceNode {
     [RaisesException=Setter] attribute AudioBuffer buffer;
 
     const unsigned short UNSCHEDULED_STATE = 0;
     const unsigned short SCHEDULED_STATE = 1;
     const unsigned short PLAYING_STATE = 2;
     const unsigned short FINISHED_STATE = 3;
 
     readonly attribute unsigned short playbackState;
 
-    readonly attribute AudioParam gain;
     readonly attribute AudioParam playbackRate;
 
     attribute boolean loop;
     attribute double loopStart;
     attribute double loopEnd;
 
     [RaisesException, MeasureAs=WebAudioStart] void start(optional double when, optional double grainOffset, optional double grainDuration);
     [RaisesException] void stop(optional double when);
 
     [RaisesException, ImplementedAs=start, MeasureAs=LegacyWebAudio] void noteOn(double when);
     [RaisesException, MeasureAs=LegacyWebAudio] void noteGrainOn(double when, double grainOffset, double grainDuration);
     [RaisesException, ImplementedAs=stop] void noteOff(double when);
 
     attribute EventHandler onended;
 };