[subresource_filter] Log histogram bucket for stronger popup blocker

This histogram is logged once every time the stronger popup blocker
blocks a popup.

A followup could aggregate this per page load if we're worried about
skew.

Bug: 756089
Change-Id: I1c2406a47fb48aeb91993ba24be485c3baf0f198
Reviewed-on: https://chromium-review.googlesource.com/633787
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497416}
4 files changed