Fix some grammatical errors in comments in tools/

tools/playback_benchmark/playback.js

 // Copyright (c) 2012 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.
 
 /**
  * @fileoverview Playback agent.
  */
 
 var Benchmark = Benchmark || {};
 
@@ skipping 71 matching lines @@
       if (event[properties[i]] != recordedEvent[properties[i]])
         equal = false;
   }
   if (!equal) {
     Benchmark.die('unexpected event: ' + JSON.stringify(event) +
                   ' instead of ' + JSON.stringify(recordedEvent));
   }
 };
 
 /**
- * Gets next event from timeline and returns it's identifier.
+ * Gets next event from timeline and returns its identifier.
  * @param {Object} event Object with event information.
  * @return {number} Event identifier.
  */
 Benchmark.Agent.prototype.createAsyncEvent = function(event) {
   return this.getNextEvent(event).id;
 };
 
 /**
  * Stores callback to be invoked according to timeline order.
  * @param {number} eventId 'Parent' event identifier.
@@ skipping 146 matching lines @@
 Benchmark.reportScore = function(score) {
   Benchmark.score = score;
 };
 
 Benchmark.originals.addEventListenerToWindow.call(
     window, 'message', function(event) {
       if (Benchmark.score) {
         event.source.postMessage(Benchmark.score, event.origin);
       }
     }, false);