This CL adds support for AncestorThrottle to parse CSP headers coming
from requests generated in the browser process, such as requests coming
from signed exchanges or AppCache.

Bug: 1042627
Change-Id: I7364cfc5b1c15181cce3e69cc254d7aea435124e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020405
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741559}