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The windows of removed desks should be demoted to be LRU across
all desks. That's why they're appended at the end of the overview
grid list. However, their relative order should be maintained.

This CL makes sure the MRU order is updated to match the above
expectation, as well as the windows' stacking order (that is
windows of the removed desk should be stacked at the bottom
of the target desk's container's children).

BUG=1048271
TEST=Expanded existing tests.

Change-Id: I400b2ac3ce2b5de1f1d8defaff7c4d155e4fab29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076670
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745202}