Add histograms for tracking header webRequest modifications

These will help us know how often these special headers are changed
or removed. Also added a histogram to track the number of users with
a webRequest extension installed.

Bug: 827582
Change-Id: Ia4e53d528363eb29a9271e6b635b97dd90117a7d
Reviewed-on: https://chromium-review.googlesource.com/c/1320469
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606267}