Update WebAudio layout tests to use unprefixed AudioContext.

LayoutTests/webaudio/resources/waveshaper-testing.js

 var context;
 var lengthInSeconds = 2;
 
 // Skip this many frames before comparing against reference to allow
 // a steady-state to be reached in the up-sampling filters.
 var filterStabilizeSkipFrames = 2048;
 
 var numberOfCurveFrames = 65536;
 var waveShapingCurve;
 
@@ skipping 136 matching lines @@
 
     if (window.testRunner) {
         testRunner.dumpAsText();
         testRunner.waitUntilDone();
     }
 
     window.jsTestIsAsync = true;
 
     // Create offline audio context.
     var numberOfRenderFrames = sampleRate * lengthInSeconds;
-    context = new webkitOfflineAudioContext(1, numberOfRenderFrames, sampleRate);
+    context = new OfflineAudioContext(1, numberOfRenderFrames, sampleRate);
 
     // source -> waveshaper -> destination
     var source = context.createBufferSource();
     source.buffer = createToneBuffer(context, fundamentalFrequency, lengthInSeconds, 1);
 
     // Apply a non-linear distortion curve.
     waveshaper = context.createWaveShaper();
     waveshaper.curve = generateWaveShapingCurve();
     waveshaper.oversample = oversample;
 
     source.connect(waveshaper);
     waveshaper.connect(context.destination);
 
     source.start(0);
 
     context.oncomplete = checkShapedCurve;
     context.startRendering();
 }