DevTools: do not show gray time for unaccounted time on threads other than main

Source/devtools/front_end/timeline/TimelineFrameModel.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 387 matching lines @@
             this._addBackgroundTraceEvent(event);
         }
     },
 
     /**
      * @param {!WebInspector.TracingModel.Event} event
      */
     _addBackgroundTraceEvent: function(event)
     {
         var eventNames = WebInspector.TimelineModel.RecordType;
-        if (this._lastFrame && event.selfTime)
+        if (this._lastFrame && event.selfTime && !WebInspector.TracingModel.isTopLevelEvent(event))
             this._lastFrame._addTimeForCategory(WebInspector.TimelineUIUtils.eventStyle(event).category.name, event.selfTime);
 
         if (event.args["layerTreeId"] !== this._layerTreeId)
             return;
 
         var timestamp = event.startTime;
         if (event.name === eventNames.BeginFrame)
             this.handleBeginFrame(timestamp);
         else if (event.name === eventNames.DrawFrame)
             this.handleDrawFrame(timestamp);
@@ skipping 233 matching lines @@
  */
 WebInspector.PendingFrame = function(timeByCategory)
 {
     /** @type {!Object.<string, number>} */
     this.timeByCategory = timeByCategory;
     /** @type {!Array.<!WebInspector.LayerPaintEvent>} */
     this.paints = [];
     /** @type {number|undefined} */
     this.mainFrameId = undefined;
 }