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This is almost entirely from Aaron Tagliaboschi's work at
https://chromium-review.googlesource.com/c/chromium/src/+/2019602

This code also removes the unconditional use of GetFrozenUserAgent in
isolated_prerender_tab_helper.cc which looked like a mistake;
::GetUserAgent uses that when the experiment is on.

Bug: 955620
Change-Id: Id4b719e0b4b1aada85418a889203bb5a85a3e72b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079466
Commit-Queue: Maksim Orlovich <morlovich@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756001}