DevTools: Add snapshot command log on the frontend.

Source/devtools/front_end/timeline/Layers3DView.js

 /*
  * Copyright (C) 2014 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 620 matching lines @@
 
         this._layerTree.forEachLayer(checkIntersection.bind(this));
         return closestLayer;
     },
 
     /**
      * @param {?Event} event
      */
     _onContextMenu: function(event)
     {
-        var layer = this._layerFromEventPoint(event).layer;
+        var layer = this._layerFromEventPoint(event) && this._layerFromEventPoint(event).layer;
         var node = layer ? layer.nodeForSelfOrAncestor() : null;
         var contextMenu = new WebInspector.ContextMenu(event);
         contextMenu.appendItem("Reset view", this._transformController._resetAndNotify.bind(this._transformController), false);
         if (node)
             contextMenu.appendApplicableItems(node);
         contextMenu.show();
     },
 
     /**
      * @param {?Event} event
@@ skipping 32 matching lines @@
     _onDoubleClick: function(event)
     {
         var object = this._layerFromEventPoint(event);
         if (object && object.layer)
             this.dispatchEventToListeners(WebInspector.Layers3DView.Events.LayerSnapshotRequested, object.layer);
         event.stopPropagation();
     },
 
     __proto__: WebInspector.VBox.prototype
 }