DevTools: [SSP] Implement adding new rule in user stylesheet

LayoutTests/http/tests/inspector/elements-test.js

 var initialize_ElementTest = function() {
 
 InspectorTest.findNode = function(matchFunction, callback)
 {
     callback = InspectorTest.safeWrap(callback);
     var result = null;
     var pendingRequests = 0;
     function processChildren(node)
     {
         try {
@@ skipping 710 matching lines @@
     var selectors = [];
     for (var i = 0; i < rule.matchingSelectors.length; ++i)
         selectors.push(rule.selectors[rule.matchingSelectors[i]].value);
     return "[" + selectors.join(", ") + "]";
 }
 
 InspectorTest.addNewRule = function(selector, callback)
 {
     // Click "Add new rule".
     document.getElementById("add-style-button-test-id").click();
-    var section = WebInspector.panels.elements.sidebarPanes.styles.sections[0][2];
-    if (typeof selector === "string")
-        section._selectorElement.textContent = selector;
-    section._selectorElement.dispatchEvent(InspectorTest.createKeyEvent("Enter"));
-    InspectorTest.runAfterPendingDispatches(callback.bind(null, section));
+    InspectorTest.addSniffer(WebInspector.StylesSidebarPane.prototype, "_addBlankSection", onBlankSection);
+
+    function onBlankSection()
+    {
+        var section = WebInspector.panels.elements.sidebarPanes.styles.sections[0][2];
+        if (typeof selector === "string")
+            section._selectorElement.textContent = selector;
+        section._selectorElement.dispatchEvent(InspectorTest.createKeyEvent("Enter"));
+        InspectorTest.runAfterPendingDispatches(callback.bind(null, section));
+    }
 }
 
 InspectorTest.dumpInspectorHighlight = function(node, callback)
 {
     node.boxModel(function(boxModel) {
         var rectNames = ["margin", "border", "padding", "content"];
         for (var i = 0; i < rectNames.length; i++) {
             var rect = boxModel[rectNames[i]];
             InspectorTest.addResult(rectNames[i] + " rect is " + (rect[4] - rect[0]) + " x " + (rect[5] - rect[1]) + " at (" + rect[0] + ", " + rect[1] + ")");
         }
         callback();
    });
 }
 
 InspectorTest.dumpInspectorHighlightShape = function(node, callback)
 {
     node.boxModel(function(shapes) {
         InspectorTest.addResult(JSON.stringify(shapes.shapeOutside.shape));
         callback();
     });
 }
 
 };