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This CL replaces cros_colors and cros_palette the new JSON5 generated
color format.

This CL also adds GN generation targets and plumbs the generated CSS
file through to OS Settings.

Bug: 1018654
Change-Id: I5ef6b3f76c48aa59eac54163b085cd555b18369a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035483
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746264}