[DevTools] Make resource tree model optional

third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js

Index: third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
diff --git a/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js b/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
index dea9998afa2fcbc9dd41d24782161bb90da64b45..5005854384c60c3013e04de51633e4920ae12068 100644
--- a/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
+++ b/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
@@ -39,7 +39,9 @@ WebInspector.CSSModel = function(target)
     this._domModel = WebInspector.DOMModel.fromTarget(target);
     this._agent = target.cssAgent();
     this._styleLoader = new WebInspector.CSSModel.ComputedStyleLoader(this);
-    target.resourceTreeModel.addEventListener(WebInspector.ResourceTreeModel.EventTypes.MainFrameNavigated, this._mainFrameNavigated, this);
+    var resourceTreeModel = WebInspector.ResourceTreeModel.fromTarget(target);
+    if (resourceTreeModel)
+        resourceTreeModel.addEventListener(WebInspector.ResourceTreeModel.EventTypes.MainFrameNavigated, this._mainFrameNavigated, this);
     target.registerCSSDispatcher(new WebInspector.CSSDispatcher(this));
     this._agent.enable().then(this._wasEnabled.bind(this));
     /** @type {!Map.<string, !WebInspector.CSSStyleSheetHeader>} */
@@ -727,7 +729,8 @@ WebInspector.CSSModel.prototype = {
      */
     requestViaInspectorStylesheet: function(node, userCallback)
     {
-        var frameId = node.frameId() || this.target().resourceTreeModel.mainFrame.id;
+        var resourceTreeModel = WebInspector.ResourceTreeModel.fromTarget(this.target());
+        var frameId = node.frameId() || (resourceTreeModel && resourceTreeModel.mainFrame.id);
         var headers = this._styleSheetIdToHeader.valuesArray();
         for (var i = 0; i < headers.length; ++i) {
             var styleSheetHeader = headers[i];