Start a first-party reporting mechanism for feature policy

This just implements the infrastructure required for generating and
sending reports with the Reporting API, and making them visible to
ReportingObservers. Adding violation reporting to individual features
will be handled in follow-up CLs.

Bug: 867471
Change-Id: Iabfaebe00ae5a768d1b952a90aac1712aec5e158
Reviewed-on: https://chromium-review.googlesource.com/924475
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582571}
12 files changed