Prepare blink to unify definitions of load completion

LayoutTests/http/tests/svg/change-id-with-pending-resources.html

 <script>
 // Test passes if it does not crash.
 // Note: this test is located under Layouttests/http in order to load an external
 //       document and modify it without hitting security restrictions.
     if (window.testRunner) {
         testRunner.waitUntilDone();
         testRunner.dumpAsText();
     }
 
     function crash() {
         q = document.getElementById('root').contentDocument;
         document.write("");
         q.getElementById('d').id = "maskedGradient";
         document.write("If this text is visible and the test did not crash, this test passes");
+        document.close();
         if (window.testRunner)
             testRunner.notifyDone();
     }
 </script>
 <object data="http://127.0.0.1:8000/svg/resources/svg-use-defs-rect.svg" id="root" onload="crash()"/>