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This CL fixes the overlap between the desks bar, and the splitview
drag indicators when in portrait tablet mode. The desks bar is now
shifted down when necessary to make room for the indicators.

BUG=998239
TEST=Manually, Added new tests.

Change-Id: Ic890a8d344a93551c617cb3f2212f19f869ab611
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787158
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693888}