Fix crash caused by render frame was gone

FormTracker::FormControlDidChangeImpl is post task when render frame
could be gone.

Bug: 820773
Change-Id: I234d4b44a27903ea2e4c4b9f83fab63ec9accb00
Reviewed-on: https://chromium-review.googlesource.com/962921
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543152}

components/autofill/content/renderer/form_tracker.cc

@@ -126,6 +126,9 @@ void FormTracker::FormControlDidChangeImpl(
     const WebFormControlElement& element,
     Observer::ElementChangeSource change_source) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(form_tracker_sequence_checker_);
+  // Render frame could be gone as this is the post task.
+  if (!render_frame()) return;
+
   if (element.Form().IsNull()) {
     last_interacted_formless_element_ = element;
   } else {