Switch feature policy reporting to an Origin Trial

This change turns feature policy reporting from an experimental feature
into an Origin Trial. The potential availability of the trial in any
document means that the Feature-policy-report-only header may be parsed
in any document, and the various policy structures created, but the
reports will only be sent through the Reporting API and to Reporting
Observer if the trial is enabled.

Bug: 867471
Change-Id: Ice5114a87ec082228e125b06df0c206c21f1e38a
Reviewed-on: https://chromium-review.googlesource.com/c/1403918
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624815}
14 files changed