Make AudioBufferSourceNode.buffer setter conforming

third_party/WebKit/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 15 matching lines @@
 // A cached (non-streamed), memory-resident audio source
 // See https://webaudio.github.io/web-audio-api/#AudioBufferSourceNode
 [
     Constructor(BaseAudioContext context, optional AudioBufferSourceOptions options),
     RaisesException=Constructor,
     ActiveScriptWrappable,
     DependentLifetime,
     Measure
 ]
 interface AudioBufferSourceNode : AudioScheduledSourceNode {
-    [RaisesException=Setter] attribute AudioBuffer buffer;
+    [RaisesException=Setter] attribute AudioBuffer? buffer;
 
     readonly attribute AudioParam playbackRate;
     readonly attribute AudioParam detune;
 
     attribute boolean loop;
     attribute double loopStart;
     attribute double loopEnd;
 
     [RaisesException] void start(optional double when = 0, optional double grainOffset, optional double grainDuration);
 
 };