Fix crash in MutationObserver::canDeliver due to NULL ExecutionContext

Rather than NULL-checking it directly, I've instead plumbed through
ActiveDOMCallback::canInvokeCallback(), which handles all the logic
MutationObserver was doing previously and also handles the NULL check.

The test case is crazy-looking because it was generated by ClusterFuzz.
I've simplified it as much as possible.

BUG=410755

Review URL: https://codereview.chromium.org/657983002

git-svn-id: svn://svn.chromium.org/blink/trunk@183827 bbb929c8-8fbe-4397-9dbb-9b2b20218538