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This CL adds the ability to customize MdTextButton's so that their
corners are rounded. This CL then makes the tab switch button round.

Bug: 780835
Change-Id: I2c46481caef074db009e681491d4f0b6822be861
Reviewed-on: https://chromium-review.googlesource.com/963836
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545693}