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This make sures requestPictureInPicture is resolved with existing
Picture-in-Picture window when video is already pictureInPictureElement.

Bug: 806249
Change-Id: I16e3fc2e8e19c2b623e8d2518fe4672a2a16b756
Reviewed-on: https://chromium-review.googlesource.com/1101686
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567934}