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The autoplay visibility observer is reset in various situations but
wasn't when the source of the video was change which broke use cases
such as playing A then switching to B. B would freeze to the first frame
and never update.

Bug: 804091
Change-Id: I984d4c5d67c575d3c45f50877d329264c1a6020e
Reviewed-on: https://chromium-review.googlesource.com/890942Reviewed-by: apacible <apacible@chromium.org>
Commit-Queue: Mounir Lamouri (slow) <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532828}