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Changes:
1. Update the optional divider ratios (0.33f, 0.67f) according to the minimum size of
   the snapped windows.
2. Show the black scrim when the divider passes over the window's minimum size.

Test: SplitViewControllerTest.DividerPositionWithWindowMinimumSizeTest
Bug: 775227
Change-Id: Ia7ca6b42639a2e8963d5bfb95eb449e520f048fa
Reviewed-on: https://chromium-review.googlesource.com/735980
Commit-Queue: min c <minch@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513222}