Inherit EventTarget interface instead of duplicating its code

Source/modules/webaudio/AudioContext.idl

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 [
     NoInterfaceObject,
     Conditional=WEB_AUDIO,
     ActiveDOMObject,
-    CustomConstructor,
-    EventTarget
-] interface AudioContext {
+    CustomConstructor
+] interface AudioContext : EventTarget {
     // All rendered audio ultimately connects to destination, which represents the audio hardware.
     readonly attribute AudioDestinationNode destination;
 
     // All scheduled times are relative to this time in seconds.
     readonly attribute float currentTime;
 
     // All AudioNodes in the context run at this sample-rate (sample-frames per second).
     readonly attribute float sampleRate;
 
     // All panning is relative to this listener.
@@ skipping 37 matching lines @@
     // void prepareOfflineBufferRendering(unsigned long numberOfChannels, unsigned long numberOfFrames, float sampleRate);
     attribute EventListener oncomplete;
     void startRendering();
 
     [MeasureAs=LegacyWebAudio, ImplementedAs=createGain] GainNode createGainNode();
     [MeasureAs=LegacyWebAudio, ImplementedAs=createDelay, RaisesException] DelayNode createDelayNode(optional double maxDelayTime);
 
     [MeasureAs=LegacyWebAudio, ImplementedAs=createScriptProcessor, RaisesException] ScriptProcessorNode createJavaScriptNode(unsigned long bufferSize, optional unsigned long numberOfInputChannels, optional unsigned long numberOfOutputChannels);
 
 };