Prepare blink to unify definitions of load completion

LayoutTests/http/tests/svg/tref-adoptNode-crash.html

 <script>
 // Test passes if it does not crash.
 // Note: this test is located under Layouttests/http in order to load an external
 //       document (svg-tref.svg) and modify it without hitting security restrictions.
     if (window.testRunner) {
         testRunner.waitUntilDone();
         testRunner.dumpAsText();
     }
 
     function crash() {
         q = document.getElementById('root').contentDocument;
         var z = document.lastChild;
         q.adoptNode( z );
         e = document.importNode( q.firstChild, true );
         q.adoptNode( e );
         r = document.createRange();
         r.surroundContents( e );
         e.id = 's';
         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-tref.svg" id="root" onload="crash()"/>