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This is needed to notify cast apps when they are covered by other
activities.

Bug: b/120809866
Test: Compile
Change-Id: I312117a7b051183ad0056cfeff2d0edff49a6f82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900388Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#713520}