DevTools: defer styles delta calculation to until the end of the task.

Source/devtools/front_end/CSSStyleModel.js

Index: Source/devtools/front_end/CSSStyleModel.js
diff --git a/Source/devtools/front_end/CSSStyleModel.js b/Source/devtools/front_end/CSSStyleModel.js
index 3d54bf465c4fafdb90d2f6f36fefc29dae0a33dc..8300f2d56369516856ea4dc3f64e51f827eaba65 100644
--- a/Source/devtools/front_end/CSSStyleModel.js
+++ b/Source/devtools/front_end/CSSStyleModel.js
@@ -345,7 +345,6 @@ WebInspector.CSSStyleModel.prototype = {
 
     mediaQueryResultChanged: function()
     {
-        this._styleLoader.reset();
         this.dispatchEventToListeners(WebInspector.CSSStyleModel.Events.MediaQueryResultChanged);
     },
 
@@ -383,7 +382,6 @@ WebInspector.CSSStyleModel.prototype = {
      */
     _fireStyleSheetChanged: function(styleSheetId)
     {
-        this._styleLoader.reset();
         if (!this._pendingCommandsMajorState.length)
             return;
 
@@ -413,7 +411,6 @@ WebInspector.CSSStyleModel.prototype = {
             frameIdToStyleSheetIds[styleSheetHeader.frameId] = styleSheetIds;
         }
         styleSheetIds.push(styleSheetHeader.id);
-        this._styleLoader.reset();
         this.dispatchEventToListeners(WebInspector.CSSStyleModel.Events.StyleSheetAdded, styleSheetHeader);
     },
 
@@ -433,7 +430,6 @@ WebInspector.CSSStyleModel.prototype = {
             if (!Object.keys(this._styleSheetIdsForURL[url]).length)
                 delete this._styleSheetIdsForURL[url];
         }
-        this._styleLoader.reset();
         this.dispatchEventToListeners(WebInspector.CSSStyleModel.Events.StyleSheetRemoved, header);
     },
 
@@ -1523,16 +1519,6 @@ WebInspector.CSSStyleModel.ComputedStyleLoader = function(cssModel)
 }
 
 WebInspector.CSSStyleModel.ComputedStyleLoader.prototype = {
-    reset: function()
-    {
-        for (var nodeId in this._nodeIdToCallbackData) {
-            var callbacks = this._nodeIdToCallbackData[nodeId];
-            for (var i = 0; i < callbacks.length; ++i)
-                callbacks[i](null);
-        }
-        this._nodeIdToCallbackData = {};
-    },
-
     /**
      * @param {!DOMAgent.NodeId} nodeId
      * @param {function(?WebInspector.CSSStyleDeclaration)} userCallback