Re: "has the spec already implemented for Windows", it's the same code path.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c301f0abb8821646d3acea02aa73425360157d86

commit c301f0abb8821646d3acea02aa73425360157d86
Author: Leonard Grey <lgrey@chromium.org>
Date: Thu Jan 10 00:32:10 2019

Update MD buttons for MD refresh

This change fixes a few things that fell through the cracks in MD refresh,
as part of the work to enable dark mode. Specifically:

- Updates text colors
- Untethers non-prominent button border from text color
- Updates disabled prominent button appearance to use a grey background,
instead of shading the enabled background

Spec: https://docs.google.com/presentation/d/1EO7TOpIMJ7QHjaTVw9St-q6naKwtXX2TwzMirG5EsKY/edit#slide=id.g34d24941d1_37_5

Bug:  918681 
Change-Id: I5ef400ed35b5c85c90c9ee3b5c33703136bdc9b0
Reviewed-on: https://chromium-review.googlesource.com/c/1399044
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621391}
[modify] https://crrev.com/c301f0abb8821646d3acea02aa73425360157d86/chrome/browser/ui/libgtkui/native_theme_gtk.cc
[modify] https://crrev.com/c301f0abb8821646d3acea02aa73425360157d86/chrome/browser/ui/views/chrome_typography_provider.cc
[modify] https://crrev.com/c301f0abb8821646d3acea02aa73425360157d86/ui/native_theme/common_theme.cc
[modify] https://crrev.com/c301f0abb8821646d3acea02aa73425360157d86/ui/native_theme/native_theme.h
[modify] https://crrev.com/c301f0abb8821646d3acea02aa73425360157d86/ui/native_theme/native_theme_dark_aura.cc
[modify] https://crrev.com/c301f0abb8821646d3acea02aa73425360157d86/ui/views/controls/button/md_text_button.cc