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The shelf config needs to be updated at the "starting" phase
of the tablet mode transition, so that they remain stable
during the animation. Otherwise, if the shelf bounds change
during the animation, the display work-area bounds will change
leading to many re-layouts, which will hurt the animation
smoothness.

BUG=1044316
TEST=Run the tast test: ui.TabletTransitionPerf and make sure
the animation smoothness is back to its normal value.

Change-Id: Ia376cba2d9ca624ee762da18e1bbe11a70257dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020616Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735169}