Define default values for options dictionaries for WebAudio

third_party/WebKit/Source/modules/webaudio/OscillatorOptions.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // See https://webaudio.github.io/web-audio-api/#oscillatoroptions
 dictionary OscillatorOptions : AudioNodeOptions {
-    OscillatorType type;
-    float detune;
-    float frequency;
+    OscillatorType type = "sine";
+    float detune = 0;
+    float frequency = 440;
     PeriodicWave? periodicWave;
 };