DevTools: allow injecting CSS rules without breaking styles sidebar.

LayoutTests/http/tests/inspector-protocol/resources/protocol-test.html

 <!--
 Copyright (C) 2012 Samsung Electronics. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 
 1.  Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 230 matching lines @@
 /**
  * Evaluates string in page.
  * @param {string} message
  * @param {!function} callback
  */
 InspectorTest.evaluateInPage = function(string, callback)
 {
     this.sendCommand("Runtime.evaluate", { "expression": string }, function(message) {
         if (message.error)
             InspectorTest.log("Error while executing '" + string + "': " + message.error.message);
-        else
+        else if (callback)
             callback();
     });
 };
 
 InspectorTest.completeTestIfError = function(messageObject)
 {
     if (messageObject.error) {
         InspectorTest.log(messageObject.error.message);
         InspectorTest.completeTest();
         return true;
@@ skipping 40 matching lines @@
 function enableInspectorAgent()
 {
     InspectorTest.sendCommand("Inspector.enable", { });
 }
 
 window.addEventListener("load", enableInspectorAgent, false);
 
 </script>
 </head>
 </html>