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This change:
1) Adds NativeTheme::UsesHighContrastColors(), which returns whether special
   high contrast colors should be used, based on system settings;
2) Adds a new --force-high-contrast switch to override the system settings;
3) Adds overrides for certain colors to use black or GoogleBlue900 as needed;
4) Adds gfx::kGoogleBlue900 from the Polymer color specs

Bug: 565996
Change-Id: Ie63f4d965a62bcfdb092660673b52534ce4507dd
Reviewed-on: https://chromium-review.googlesource.com/761316Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523846}