GitLab

This cl:
1) Moves a bunch of tests from WindowSelectorTest to new
ScopedTransformOverviewWindowTest file.
2) Moves some static class constants to a newly introduced overview
constants file.

Test: ash_unittests ScopedTransformOverviewWindowTest*
Bug: 914147
Change-Id: I1c81533a0d0cb245c05ce4a4d288b574c0fc1784
Reviewed-on: https://chromium-review.googlesource.com/c/1394324
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619799}