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Mostly adds testing to ensure the UA header is set correctly, and uses
the content API for the UA string instead.

Change-Id: Id54ec973dc6837b92aa19cde3149e1097423374d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521351
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Auto-Submit: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824555}