Fix flakiness and re-enable FormAutocompleteTest

extensions/renderer/dispatcher.cc

Index: extensions/renderer/dispatcher.cc
diff --git a/extensions/renderer/dispatcher.cc b/extensions/renderer/dispatcher.cc
index 66ffdcfa9d3ed41c801c351eb6933b20543c229c..5e29617847d58f99d3c47cdfeeb7349ed819efc8 100644
--- a/extensions/renderer/dispatcher.cc
+++ b/extensions/renderer/dispatcher.cc
@@ -361,7 +361,12 @@ void Dispatcher::DidCreateDocumentElement(blink::WebLocalFrame* frame) {
                                 .as_string()));
   }
 
-  content_watcher_->DidCreateDocumentElement(frame);
+  // In testing, the document lifetime events can happen after the render
+  // process shutdown event.
+  // See: http://crbug.com/21508 and http://crbug.com/500851
+  if (content_watcher_) {
+    content_watcher_->DidCreateDocumentElement(frame);
+  }
 }
 
 void Dispatcher::OnExtensionResponse(int request_id,