Reland "Prepare blink to unify definitions of load completion"

LayoutTests/fast/parser/adoption-agency-crash-01.html

 <script>
 if (window.testRunner)
     testRunner.dumpAsText();
 
-document.write('<a><p><iframe onload="document.write(\'<script>void(0)<\/script></a>\');"></iframe><script>document.body.innerHTML = \'PASS\';<\/script>');
+function finish() {
+    document.write('<script>void(0)<\/script></a>');
+    document.close();
+};
+
+document.write('<a><p><iframe onload="document.open(); setTimeout(finish, 0);"></iframe><script>document.body.innerHTML = \'PASS\';<\/script>');
 </script>