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Second CL to allow chaining keyboard shortcuts.

Follow up to crrev.com/c/2354798 which allows RootWindowDeskSwitch to
handle more than two screenshots. This CL fills it out by handling
taking third and fourth screenshots and placing them on the animation
layer to get a nice smooth continuous animation. Tests in a follow up.

Test: manual
Bug: 1111445, 1068508
Change-Id: I33f4a26ae794fca799888953b23d617a9f8d0832
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376418
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803583}