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Overview will show all windows related by transient on the same
thumbnail. Currently we will get blank spaces since the MRU list may
contain two windows related by transient, and the transient ancestor
or descendant of the most recent window will get transformed onto the
transient related windows thumbnail, leaving a blank space.

Introduce a function which removes all transient descendants from a
given list if their transient root exists in the same list. First
usage is overview but may need to extend to alt tab in future.

Test: ash_unittests WindowUtilTest.RemoveTransientDescendants
Bug: 844829
Change-Id: Ic2f9b476bc353b50a197be8fcf7bb3f2e3d44c49
Reviewed-on: https://chromium-review.googlesource.com/c/1355249
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612535}