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The custom layout used by ComboxBox simply calls the parent's layout
function and set up the arrow button's bounds. The latter can be
replaced in OnBoundsChanged(). After that move, the custom layout is no
longer needed.

BUG=1005568

Change-Id: Ib3ea796fbaa6bf626ad5d07e00d6db8c2fbc31b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890976Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711366}