Ship the `Sec-Fetch-{Mode,Site,User}` request headers.

Spec: https://mikewest.github.io/sec-metadata/
Intent to Ship: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/yQgJlq5PEOQ/erexYRWHBgAJ

Bug: 843478
Change-Id: Iae3bbaa9c908f9f8f281cd97ba94b1601556cb93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547515
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652776}
11 files changed