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This way the embedder can readily detect when and if a Browser
or Tab has been destroyed.

BUG=1134085
TEST=covered by tests

Change-Id: I2e316e023d18c04d4aec6139306d1815e1ee65cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443402Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813009}