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The tab strip uses the BottomToolbarButton style. The label CL modified
that style and it broke the tab strip buttons. This CL reverts the
BottomToolbarButton style to what it used to be and introduces a new
style, SplitToolbarButton, for use in the split toolbar.

Bug: 966355, 966356, 966357
Change-Id: I83f53649fad8e109d0a267eb7fb26e8e4e5c286e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625966Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662810}