Implement `Sec-CH-UA-*` replacements for `User-Agent`.

This is a first pass that more or less completely implements the hints.
There's still a reasonable amount of work to be done around the edges,
like allowing devtools overrides, and getting the data delivered in the
new headers to be exactly what it ought to be. Small steps!

Intent to Implement: https://groups.google.com/a/chromium.org/d/msg/blink-dev/WQ0eC_Gf8bw/dhWMhCYYDwAJ

Bug: 928669
Change-Id: I36081a7864c9c40f6f46652477a32f5d90caaed9
Reviewed-on: https://chromium-review.googlesource.com/c/1469941
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632581}
25 files changed