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Sometimes the screen width can be odd numbered, and the current
logic will end up with a slight gap between the snapped bounds.

This CL fixes the logic, the width will be based on the mid point
of the work area.

Bug: 1020351
Test: the new test case
Change-Id: I8d4e3a7cac6ecf5ab4b18e5f6f5404223e88c9e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902766Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713501}