Add support for onvoiceschanged event.

Source/modules/speech/SpeechSynthesis.idl

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 [
     NoInterfaceObject,
     RuntimeEnabled=SpeechSynthesis
-] interface SpeechSynthesis  {
+] interface SpeechSynthesis : EventTarget {
     readonly attribute boolean pending;
     readonly attribute boolean speaking;
     readonly attribute boolean paused;
 
     [RaisesException] void speak(SpeechSynthesisUtterance utterance);
     void cancel();
     void pause();
     void resume();
     sequence<SpeechSynthesisVoice> getVoices();
+
+    attribute EventHandler onvoiceschanged;
 };