GitLab

This CL:
1. Allows chrome to add policy headers for navigations and redirects
   via content API.
2. Removes the old |PolicyHeaderIOHelper| codepath and moves policy
   header tests to a new location.
3. Adds a move constructor/assignment for |net::HttpRequestHeaders|.

Bug: 832749,845683
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I713ed30da33ba5feb5f437f8927b173c2f84d91a
Reviewed-on: https://chromium-review.googlesource.com/1100393
Commit-Queue: Chong Zhang <chongz@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Drew Wilson <atwilson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569503}