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Requested in crrev.com/c/2354798. This will simplfy things down the line
as we use more index lookup and remove const from some parameters in
RootWindowDeskSwitchAnimator.

Test: manual, existing tests
Bug: 1111445
Change-Id: I7250d969b95f08da5819c73ca325bce2db0ea5bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369518Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800828}