Only look at the last 10K samples for WebRTC muting tests.

content/test/data/media/webrtc_test_audio.js

 // Copyright 2013 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.
 
 // Audio test utilities.
 
 // GetStats reports audio output energy in the [0, 32768] range.
 var MAX_AUDIO_OUTPUT_ENERGY = 32768;
 
 // Queries WebRTC stats on |peerConnection| to find out whether audio is playing
@@ skipping 116 matching lines @@
 
   var expectedPeaks = 3;
   console.log(numPeaks + '/' + expectedPeaks + ' signal peaks identified.');
   return numPeaks >= expectedPeaks;
 }
 
 /**
  * @private
  */
 function identifySilence_(samples) {
+  // Look at the last 10K samples only to make detection a bit faster.
+  var window = samples.slice(-10000);
+
   var average = 0;
-  for (var i = 0; i < samples.length; ++i)
-    average += samples[i] / samples.length;
+  for (var i = 0; i < window.length; ++i)
+    average += window[i] / window.length;
 
   // If silent (like when muted), we should get very near zero audio level.
-  console.log('Average audio level: ' + average);
+  console.log('Average audio level (last 10k samples): ' + average);
 
   return average < 0.01 * MAX_AUDIO_OUTPUT_ENERGY;
 }
 
 /**
  * @private
  */
 function getAudioLevelFromStats_(response) {
   var reports = response.result();
   var audioOutputLevels = [];
   for (var i = 0; i < reports.length; ++i) {
     var report = reports[i];
     if (report.names().indexOf('audioOutputLevel') != -1) {
       audioOutputLevels.push(report.stat('audioOutputLevel'));
     }
   }
   return audioOutputLevels;
 }