Inherit EventTarget interface instead of duplicating its code

Source/modules/webaudio/OfflineAudioContext.idl

 /*
  * Copyright (C) 2012, Google Inc. All rights reserved.
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 [
     NoInterfaceObject,
     Conditional=WEB_AUDIO,
-    EventTarget,
     Constructor(unsigned long numberOfChannels, unsigned long numberOfFrames, float sampleRate),
     ConstructorRaisesException,
     ConstructorCallWith=ScriptExecutionContext
 ] interface OfflineAudioContext : AudioContext {
 
 };