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This change adds a high-contrast theme supplier (IncreasedContrastThemeSupplier)
and enables it on Mac when the system "Increase Contrast" setting is enabled,
or when --force-high-contrast is passed at startup. This theme maximizes
contrast between UI elements.

Bug: 831769
Change-Id: I6cbb1b67fd95372dd65501feab6639942f767019
Reviewed-on: https://chromium-review.googlesource.com/1142030Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576467}