themes: expand --force-high-contrast to other platforms

This lets users of Windows/Linux supply this flag to force the high contrast
appearance. There is no automated high contrast on those platforms yet.

Bug: 831769
Change-Id: I5dc927857db632999944e3ac8b3ae4b134dfe0be
Reviewed-on: https://chromium-review.googlesource.com/1143695
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576601}
1 file changed