Support counter graphs in Timeline based on trace events

Source/devtools/front_end/timeline/TracingTimelineModel.js

Index: Source/devtools/front_end/timeline/TracingTimelineModel.js
diff --git a/Source/devtools/front_end/timeline/TracingTimelineModel.js b/Source/devtools/front_end/timeline/TracingTimelineModel.js
index cb302fb76326803f2cf441558a18457d7937d9fc..77d227d97e2f631d4d44f34f578983053ba893a8 100644
--- a/Source/devtools/front_end/timeline/TracingTimelineModel.js
+++ b/Source/devtools/front_end/timeline/TracingTimelineModel.js
@@ -607,6 +607,14 @@ WebInspector.TracingTimelineModel.TraceEventRecord.prototype = {
     /**
      * @return {?Object}
      */
+    counters: function()
+    {
+        return this.type() === WebInspector.TracingTimelineModel.RecordType.UpdateCounters ? this._event.args.data : null;
+    },
+
+    /**
+     * @return {?Object}
+     */
     highlightQuad: function()
     {
         return this._event.highlightQuad || null;