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An observer of OnWindowActivating or OnWindowActivated may destroy
the window losing, or gaining activation.
The FocusController should handle these cases, to avoid crashes, and/or
returning dangling pointers when querying the active window.

BUG=1048870
TEST=Expanded an existing test, which crashes without the fix.

Change-Id: I804210e2f3250f69fc02b9a93eaee5d0236fca5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2038131
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739288}