Avoid trigger compositing update on media player finalization.

When finalizing a media element and clearing out its associated
media player, prevent the media player finalizer from indirectly
triggering a composition update on the media element itself.
Unsafe, as it will touch other heap objects, which might have
been finalized by now.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/696103003

git-svn-id: svn://svn.chromium.org/blink/trunk@184769 bbb929c8-8fbe-4397-9dbb-9b2b20218538