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This is basically Aaron Tagliaboschi's:
https://chromium-review.googlesource.com/c/chromium/src/+/2012902
just with a bit of additional updated of tests.

See https://github.com/WICG/ua-client-hints/issues/15 for rationale.

Change-Id: I6562cf1a78ceb4eff18baf793cb89143782f9b7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2047410
Commit-Queue: Maksim Orlovich <morlovich@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741152}