Oilpan: HTMLMediaElement should not dispatch event while document is being disposed.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@168346 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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