DevTools: Split creating inspector stylesheet and adding a new rule into stylesheet in protocol.

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 651 matching lines @@
 }
 
 InspectorTest.matchingSelectors = function(rule)
 {
     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.addSniffer(WebInspector.BlankStylePropertiesSection.prototype, "makeNormal", callback);
+}
+
 };
 
 function dumpInspectorHighlightRects()
 {
     var rectNames = ["margin", "border", "padding", "content"];
     var rects = window.internals.inspectorHighlightRects(document);
     for (var i = 0; i < rects.length; i++)
     {
         var rectName = (i < rectNames.length ? rectNames[i] : "untitled");
         var rect = rects.item(i);
         output(rectName + " rect is " + rect.width + " x " + rect.height + " at (" + rect.left + ", " + rect.top + ")");
     }
 }