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This CL adds protobuf generation for semantic colors for Longform for use
in Google3. Generated values are provided in proto json format.

Bug: 1018654
Change-Id: I4f6648e41ab7fb1b205101bdb31362fd99ffb557
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558100
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836972}