Cup request sender: use x-cup-server-proof header when available.

After this change, the RequestSender is choosing between
x-cup-server-proof and etag value, and gives priority to the
x-cup-server-proof value if it is non-empty.

The values for both headers is read by the network stacks, and made
them available to the RequestSender, which implements the validation
mechanism in one place.

It's impractical to unit test this code, since the RequestSender
hardcodes the production public key and it can only transact CUP
with a production backend.

Bug: 606958
Change-Id: I5b0f2e9f2ba1e75ca4fd4c4becde848846541bfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2266918
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: S. Ganesh <ganesh@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782987}