Update WebAudio layout tests to use unprefixed AudioContext.

LayoutTests/webaudio/resources/oscillator-testing.js

 // Notes about generated waveforms:
 //
 // QUESTION: Why does the wave shape not look like the exact shape (sharp edges)?
 // ANSWER: Because a shape with sharp edges has infinitely high frequency content.
 // Since a digital audio signal must be band-limited based on the nyquist frequency (half the sample-rate)
 // in order to avoid aliasing, this creates more rounded edges and "ringing" in the
 // appearance of the waveform.  See Nyquist-Shannon sampling theorem:
 // http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem
 //
 // QUESTION: Why does the very end of the generated signal appear to get slightly weaker?
 // ANSWER: This is an artifact of the algorithm to avoid aliasing.
 
 var sampleRate = 44100.0;
 var nyquist = 0.5 * sampleRate;
 var lengthInSeconds = 4;
 var lowFrequency = 10;
 var highFrequency = nyquist + 2000; // go slightly higher than nyquist to make sure we generate silence there
 var context = 0;
 
 var OSC = {
     SINE: 0,
     SQUARE: 1,
     SAWTOOTH: 2,
     TRIANGLE: 3,
     CUSTOM: 4,
 };
 
 function generateExponentialOscillatorSweep(oscillatorType) {
     // Create offline audio context.
-    context = new webkitOfflineAudioContext(1, sampleRate * lengthInSeconds, sampleRate);
+    context = new OfflineAudioContext(1, sampleRate * lengthInSeconds, sampleRate);
 
     var osc = context.createOscillator();
     if (oscillatorType == OSC.CUSTOM) {
         // Create a simple waveform with three Fourier coefficients.
         // Note the first values are expected to be zero (DC for coeffA and Nyquist for coeffB).
         var coeffA = new Float32Array([0, 1, 0.5]);
         var coeffB = new Float32Array([0, 0, 0]);        
         var wave = context.createPeriodicWave(coeffA, coeffB);
         osc.setPeriodicWave(wave);
     } else {
@@ skipping 10 matching lines @@
 
     var nyquist = 0.5 * sampleRate;
     osc.frequency.setValueAtTime(10, 0);
     osc.frequency.exponentialRampToValueAtTime(highFrequency, lengthInSeconds);
 
     context.oncomplete = finishAudioTest;
     context.startRendering();    
 
     testRunner.waitUntilDone();
 }