Increase scope of URLLoader::FollowRedirect() API implementation.

URLLoader::FollowRedirect() takes two arguments: |removed_headers| and
|modified_headers|. This is partially implemented. This CL is essentially
plumbing to improve support for this two arguments.

Notable changes:

1) |modified_headers| was exposed from content/public, now
   |removed_headers| is exposed as well. Tests are added.

2) In particular, NavigationURLLoader and ThrottlingURLLoader now
   supports |removed_headers|.

[email protected],[email protected]

Bug: 911105
Change-Id: I46916d98a3900c331b25bd75eb51c21aef3e1533
Reviewed-on: https://chromium-review.googlesource.com/c/1369856
Commit-Queue: Arthur Sonzogni <[email protected]>
Reviewed-by: Misha Efimov <[email protected]>
Reviewed-by: Camille Lamy <[email protected]>
Reviewed-by: Matt Menke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#620011}
40 files changed