DevTools: add extension API to contribute trace events to timeline

third_party/WebKit/Source/devtools/front_end/timeline/MemoryCountersGraph.js

 /*
  * Copyright (C) 2013 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 62 matching lines @@
         if (counter)
           counter.appendSample(event.startTime, counters[name]);
       }
 
       var gpuMemoryLimitCounterName = 'gpuMemoryLimitKB';
       if (gpuMemoryLimitCounterName in counters)
         this._gpuMemoryCounter.setLimit(counters[gpuMemoryLimitCounterName]);
     }
     this.scheduleRefresh();
   }
+
+  /**
+   * @override
+   */
+  extensionDataAdded() {
+  }
 };