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This CL adds 'poster' to the list of attributes that the
MediaElementMutationCallback listens for. When changed, the CSS classes
of the media controls are updated. This fixes a bug where a poster added
after page load would not show up because it was hidden behind the
default poster.

Bug: 814956
Change-Id: I4b39e6995ddb35829d886e138fe8a1d573dd8036
Reviewed-on: https://chromium-review.googlesource.com/965589Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543767}