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This makes sure enterpictureinpicture and leavepictureinpicture events
are fired when user enters and leaves Picture-in-Picture from native
video controls.

Bug: 806249
Change-Id: I8d8717ce6fa97c3882913e73d8599cede6ec92dc
Reviewed-on: https://chromium-review.googlesource.com/1029951
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557978}