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- Add a tab slider and update windows shown in Alt-Tab so that
"Current desk" mode only display windows in the active desk, while
"All desks" display all windows.
- Handle special cases: As before, entering Alt-Tab with less than two
windows aren't allowed. However, if users switch from the all-desks mode
to the current-desk mode resulting in zero or one window,
it should show "no recent items" or show a window consecutively.
- Hide behind Bento flag and make sure to overwrite
kLimitAltTabToActiveDesk in case of conflict, so the old flag works.
- Disable remapping Alt+LeftClick to a right click when using Alt-Tab.
- Ash unittests for switching between modes and special cases.

Bug: 1142726
Test: A manual test (a video in crbug) and unit tests
`ash_unittests --gtest_filter=ModeSelectionWindowCycleControllerTest.*`

Change-Id: Ia203358d0b658529a3090a292ae36850ea60a36a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581684Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Cattalyya Nuengsigkapian <cattalyya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838709}