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

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

Index: LayoutTests/http/tests/inspector/elements-test.js
diff --git a/LayoutTests/http/tests/inspector/elements-test.js b/LayoutTests/http/tests/inspector/elements-test.js
index 1a969179c196feead1957add3bca67a962b7eca9..c2a5b3865ab3fbfdde20231b5d9115cee19c0ca8 100644
--- a/LayoutTests/http/tests/inspector/elements-test.js
+++ b/LayoutTests/http/tests/inspector/elements-test.js
@@ -728,11 +728,16 @@ 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)