Update WebAudio layout tests to use unprefixed AudioContext.

LayoutTests/webaudio/resources/scriptprocessornode-testing.js

 // For the current implementation of JavaScriptAudioNode, when it works with OfflineAudioContext (which runs much faster
 // than real-time) the event.inputBuffer might be overwrite again before onaudioprocess ever get chance to be called.
 // We carefully arrange the renderLengthInFrames and bufferSize to have exactly the same value to avoid this issue.
 var renderLengthInFrames = 512;
 var bufferSize = 512;
 
 var context;
 
 function createBuffer(context, numberOfChannels, length) {
     var audioBuffer = context.createBuffer(numberOfChannels, length, sampleRate);
@@ skipping 47 matching lines @@
                     return false;
             }
         }
     }
     return true;
 }
 
 function runJSNodeTest()
 {
     // Create offline audio context.
-    context = new webkitOfflineAudioContext(2, renderLengthInFrames, sampleRate);
+    context = new OfflineAudioContext(2, renderLengthInFrames, sampleRate);
 
     var sourceBuffer = createBuffer(context, sourceChannels, renderLengthInFrames);
 
     var bufferSource = context.createBufferSource();
     bufferSource.buffer = sourceBuffer;
 
     var scriptNode = context.createScriptProcessor(bufferSize, inputChannels, outputChannels);
 
     bufferSource.connect(scriptNode);
     scriptNode.connect(context.destination);
     scriptNode.onaudioprocess = processAudioData;
 
     bufferSource.start(0);
     context.oncomplete = finishJSTest;
     context.startRendering();
 }