Update WebAudio layout tests to use unprefixed AudioContext.

LayoutTests/webaudio/resources/audio-codec-test.js

 var defaultSampleRate = 44100.0;
 var lengthInSeconds = 1;
 
 var context = 0;
 var bufferLoader = 0;
 
 // Run test by loading the file specified by |url|.  An optional sample rate can be given to
 // select a context with a different sample rate.  The default value is |defaultSampleRate|.
 function runDecodingTest(url, optionalSampleRate) 
 {
     if (!window.testRunner)
         return;
 
     var sampleRate = (typeof optionalSampleRate === "undefined") ? defaultSampleRate : optionalSampleRate;
 
     // Create offline audio context.
-    context = new webkitOfflineAudioContext(1, sampleRate * lengthInSeconds, sampleRate);
+    context = new OfflineAudioContext(1, sampleRate * lengthInSeconds, sampleRate);
     
     bufferLoader = new BufferLoader(
         context,
         [ url ],
         finishedLoading
     );
     
     bufferLoader.load();
     testRunner.waitUntilDone();
 }
 
 function finishedLoading(bufferList)
 {
     testRunner.setAudioData(createAudioData(bufferList[0]));
     testRunner.notifyDone();
 }