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This CL includes:
1. Make OverlayWindowAndroid an WindowAndroid observer so that it
can observe compositor state.
2. Add a Tab observer to PictureInPictureActivity so that when the
tab that initiate PiP is destroyed or Chrome instance is closed,
PictureInPictureActivity is notified and close properly.

Bug: 964378
Change-Id: Iaab2ee5b1be3acf14d7380588aae63b0d666ec6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717237Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683246}