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This CL ensures the correct layout of the launcher
button and the overflow button in the Ash MD shelf
when the shelf is being resized. This can occur in
two situations: when the shelf is being 'stretched'
toward the center of the screen by touch, or
during a shelf hide/show animation.

BUG=668230
TEST=manual

Review-Url: https://codereview.chromium.org/2598223002
Cr-Commit-Position: refs/heads/master@{#442031}