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This also refactors the content/ accept header code to be in public
along with the signed exchange accept header suffix.

Bug: 1138648
Change-Id: Idf3f5d4f8b3034e301318bdb29d509ee46bc3d10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533331
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828037}