Add use counter that tracks whether multiple report endpoints are used

In order to help make a decision if the new reporting api should support
multiple endpoints, I've added an use counter to see how frequently the
current report-uri directive is used with multiple reporting endpoints
Spec: https://wicg.github.io/reporting/

Bug: 726634
Change-Id: I47353b559a2f57a022b2a5300ea5e2cdb88e0677
Reviewed-on: https://chromium-review.googlesource.com/563378
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485243}
3 files changed