Fix crash in MutationObserver::canDeliver due to NULL ExecutionContext

LayoutTests/fast/dom/MutationObserver/mutation-callback-dead-context-crash.html

+<div id=test><iframe src="#test" onload="setTimeout(crash, 0)"></iframe></div>
+<script>
+if (window.testRunner) {
+  testRunner.dumpAsText();
+  testRunner.waitUntilDone();
+}
+var observer = new MutationObserver(function() {});
+observer.observe(document.querySelector('#test'), { attributes: true });
+function crash() {
+  if (window.top === window) {
+    var iframe = document.querySelector('iframe');
+    var otherDocument = document.implementation.createDocument("http://www.w3.org/1999/xhtml", "html");
+    otherDocument.adoptNode(iframe.contentDocument.documentElement);
+    otherDocument.documentElement.appendChild(iframe);
+    gc();
+    setTimeout(function() {
+      console.log('Test passes if it prints this message without crashing');
+      testRunner.notifyDone();
+    }, 0);
+  }
+}
+</script>