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This CL makes sure PictureInPictureWindow is accessible from the
leavepictureinpicture event so that users don't have to keep a reference
to the PictureInPictureWindow object they got from the
enterpictureinpicture event.

Spec change: https://github.com/w3c/picture-in-picture/pull/189
Intent to ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/gwaEnJjtfKw

Change-Id: Ibb6972596cd6b766bc2ed35cc6e360b7907c594d
Bug: 1096350
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241537Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#779711}