Oilpan: HTMLMediaElement should not dispatch event while document is being disposed.

After haraken's change, Event is a heap-allocated object. Events should not be dispatched while document is being disposed.

BUG=None

Review URL: https://codereview.chromium.org/182383005

git-svn-id: svn://svn.chromium.org/blink/trunk@168346 bbb929c8-8fbe-4397-9dbb-9b2b20218538