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Work area is not always starts from the top of the display, e.g, docked
magnifier will put a view port window at the top area of the display.
This will cause that in portrait screen orientation, the height of the
work area is not always larger than the width of the work area. This cl
tried to fix this by updating the divider position on
|divider_closest_ratio_| instead on relying on the longer side of the
work area.
This cl also fixed the wrong bounds of the preview area, which was set
in its own view coordinate. So should special consider the case that
work area is not from the top of the display too.

Bug: 866680, 897900
Change-Id: I86a98aee52ecbd706385cda0a639e5182997914b
Reviewed-on: https://chromium-review.googlesource.com/c/1292833
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602364}