Prepare blink to unify definitions of load completion

LayoutTests/http/tests/misc/SVGFont-delayed-load.html

 <p>This tests that the onload handler for an iframe is delayed by a pending SVGFont load.</p>
 <pre id="console"></pre>
 <script>
     if (window.testRunner)
         testRunner.dumpAsText();
 
     function log(msg)
     {
         document.getElementById('console').appendChild(document.createTextNode(msg + "\n"));
     }
 
     var onloadFired = false;
 
     iframe = document.createElement('iframe');
     iframe.onload = function() { 
         onloadFired = true;
     };
     iframe.src = "resources/SVGFont-delayed-loader.svg";
     document.body.appendChild(iframe);
 
     function contentLoaded()
     {
         if (onloadFired)
             log("FAIL: the onload event fired before the SVGFont load finished");
         else
             log("PASS: the onload event did not fire.");
 
         // Stop the infinite load.
-        iframe.contentWindow.stop();
+        stop();
     }
 </script>