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This changes follow the spec with regards to pausing a video when it
leaves a document: when a video leaves a document, the removal process
should continue and, asynchronously, Blink checks that the video is
still in an active document. It allows a video to move from one part of
the document to another without pausing. Moving into a document or
into two different outside of the document trees will also not pause.

This is following the specification and Edge/Safari/Firefox
implementations.

Bug: 382879, 490511
Change-Id: I48abdaa5f81790e2cae7edc7b0bfb444f6ee7787
Reviewed-on: https://chromium-review.googlesource.com/c/1074808
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597242}