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Since r478352 SecondaryUI buttons on Mac have all been MdTextButtons,
so we don't really care how regular LabelButtons look on Mac.

Remove some mac-specific logic from PlatformStyle and the
label_button_unittest.cc to make these tests pass. Some tests
currently fail just because the text color changes from solid
black/white to something different under MD due to logic in
ui::NativeTheme

Bug: 713030
Change-Id: I3b925918024d6c260da007ceb96900a2afb557a4
Reviewed-on: https://chromium-review.googlesource.com/741401Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512704}