DevTools: expose layer compositing reasons and paint count via layer tree agent

Source/devtools/front_end/LayerTreeModel.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 79 matching lines @@
             this._pendingRequestLayersCallbacks.push(callback);
             return;
         }
         this._pendingRequestLayersCallbacks = [];
         this._pendingRequestLayersCallbacks.push(callback);
         function onGetLayers(error, layers)
         {
             this._root = null;
             if (error) {
                 console.error("LayerTreeAgent.getLayers(): " + error);
-                return;
+                layers = [];
             }
             this._repopulate(layers);
             for (var i = 0; i < this._pendingRequestLayersCallbacks.length; ++i)
                 this._pendingRequestLayersCallbacks[i]();
             delete this._pendingRequestLayersCallbacks;
         }
         function onDocumentAvailable()
         {
             LayerTreeAgent.getLayers(undefined, onGetLayers.bind(this))
         }
@@ skipping 50 matching lines @@
  * @constructor
  * @param {LayerTreeAgent.Layer} layerPayload
  */
 WebInspector.Layer = function(layerPayload)
 {
     this._reset(layerPayload);
 }
 
 WebInspector.Layer.prototype = {
     /**
-     * @return {string?}
+     * @return {string}
      */
     id: function()
     {
         return this._layerPayload.layerId;
     },
 
     /**
      * @return {string?}
      */
     parentId: function()
@@ skipping 103 matching lines @@
     anchorPoint: function()
     {
         return [
             this._layerPayload.anchorX || 0,
             this._layerPayload.anchorY || 0,
             this._layerPayload.anchorZ || 0,
         ];
     },
 
     /**
+     * @return {number}
+     */
+    paintCount: function()
+    {
+        return this._layerPayload.paintCount;
+    },
+
+    /**
+     * @param {function(Array.<string>?)} callback
+     */
+    requestCompositingReasons: function(callback)
+    {
+        /**
+         * @param {?string} error
+         * @param {?Array.<string>} compositingReasons
+         */
+        function callbackWrapper(error, compositingReasons)
+        {
+            if (error) {
+                console.error("LayerTreeAgent.reasonsForCompositingLayer(): " + error);
+                callback(null);
+                return;
+            }
+            callback(compositingReasons);
+        }
+        LayerTreeAgent.compositingReasons(this.id(), callbackWrapper.bind(this));
+    },
+
+    /**
      * @param {LayerTreeAgent.Layer} layerPayload
      */
     _reset: function(layerPayload)
     {
         this._children = [];
         this._parent = null;
         this._layerPayload = layerPayload;
     }
 }
 
 /**
  * @constructor
  * @implements {LayerTreeAgent.Dispatcher}
  * @param {WebInspector.LayerTreeModel} layerTreeModel
  */
 WebInspector.LayerTreeDispatcher = function(layerTreeModel)
 {
     this._layerTreeModel = layerTreeModel;
 }
 
 WebInspector.LayerTreeDispatcher.prototype = {
     layerTreeDidChange: function()
     {
         this._layerTreeModel._layerTreeChanged();
     }
 }