Support counter graphs in Timeline based on trace events

Source/devtools/front_end/timeline/TimelineModel.js

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 393 matching lines @@
      * @return {?Object}
      */
     highlightQuad: function() { },
 
     /**
      * @return {string}
      */
     type: function() { },
 
     /**
+     * @return {?Object}
+     */
+    counters: function() { },
+
+    /**
      * @return {string}
      */
     frameId: function() { },
 
     /**
      * @return {?Array.<!ConsoleAgent.CallFrame>}
      */
     stackTrace: function() { },
 
     /**
@@ skipping 80 matching lines @@
         function recordTimestampComparator(a, b)
         {
             // Never return 0, as the merge function will squash identical entries.
             return a.startTime() < b.startTime() ? -1 : 1;
         }
         var result = this._backgroundRecordsBuffer.mergeOrdered(records, recordTimestampComparator);
         this._backgroundRecordsBuffer = [];
         return result;
     }
 }