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This reverts commit f51b93d7.

Reason for revert: Suspecting as cause of crbug.com/1123786.

Original change's description:
> [Cast MRP] Stop mirroring immediately when a the tab is closed.
> 
> Bug: 1095764, b/159154368
> Change-Id: I36829467e3a6477499c76ae0b15e110acba00c89
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346911
> Reviewed-by: Yuri Wiitala <miu@chromium.org>
> Commit-Queue: John Williams <jrw@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#803343}

TBR=miu@chromium.org,dominickn@chromium.org,jrw@chromium.org

Change-Id: I3c8e562ee39e8cbf7ef5a5bb0312c9d51da7acc3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1095764
Bug: b/159154368
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386439Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803377}