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This refactors client hints to allow main frame prefetch navigations to
add client hints without needing a frame.

Related headers CL: https://chromium-review.googlesource.com/c/chromium/src/+/2552723

Bug: 1142074
Change-Id: I662d4a8529f7546df80d81128d7df8545717fbb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551671Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831974}