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The crash used to happen when a transient popup child is
added to a window while alt-tab window cycling is active,
and cycling stops before that transient is removed.

This CL fixes the crash and adds a test for it.

BUG=1014543
TEST=Added a new test that crashes without the fix.

Change-Id: Ib3abf9aee9332859ea00e6a54d3f8fc93d00449f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885496
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710188}