[DevTools] Fix race in testDeviceMetricsOverrides.

Source/devtools/front_end/common/Throttler.js

 // Copyright 2014 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.
 
 /**
  * @constructor
  * @param {number} timeout
  */
 WebInspector.Throttler = function(timeout)
 {
     this._timeout = timeout;
     this._isRunningProcess = false;
     this._asSoonAsPossible = false;
     /** @type {?function(!WebInspector.Throttler.FinishCallback)} */
     this._process = null;
 }
 
 WebInspector.Throttler.prototype = {
     _processCompleted: function()
     {
         this._isRunningProcess = false;
         if (this._process)
             this._innerSchedule(false);
+        this._processCompletedForTests();
+    },
+
+    _processCompletedForTests: function()
+    {
+        // For sniffing in tests.
     },
 
     _onTimeout: function()
     {
         delete this._processTimeout;
         this._asSoonAsPossible = false;
         this._isRunningProcess = true;
 
         // Process might issue synchronous calls to this throttler.
         var process = this._process;
@@ skipping 45 matching lines @@
      * @return {number}
      */
     _setTimeout: function(operation, timeout)
     {
         return setTimeout(operation, timeout);
     }
 }
 
 /** @typedef {function()} */
 WebInspector.Throttler.FinishCallback;