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This matches the logic in http://crrev.com/c/1045025. A couple of the
most recent configs are kept around in case the config changes in the
middle of a request, which will allow the headers to be correct in that
case.

Also adds a browsertest for basic config changes.

Bug: 721403
Change-Id: I788dcf52efa9a86031fd33cbc77d1be45022b19c
Reviewed-on: https://chromium-review.googlesource.com/c/1292510
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601701}