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This CL fixes a tab crash that occurs when calling document.exitPictureInPicture()
if a video in a shadow DOM is already in Picture-in-Picture.

Bug: 904828

Change-Id: I6aed0fa43dd0af1406d4349509101d423f622d96
Reviewed-on: https://chromium-review.googlesource.com/c/1333769Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#607658}