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Reducing the font weight only on the default white background for CrOS
and MacOS.

Screencast:
https://bugs.chromium.org/p/chromium/issues/detail?id=867779#c18

Bug: 867779
Change-Id: Ib349bd2ca7dc91e506c5bd485b1e94a1ea57616c
Reviewed-on: https://chromium-review.googlesource.com/1199809
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588713}