Add methods for manipulating policies under construction

This is used immediately for clearing up the logic around allowfullscreen
and allowpayment request, and can also be used for more complex
container policy logic in the future.

Change-Id: I9cc702e0344d86d56a91894dc7a5a25bcdc26424
Reviewed-on: https://chromium-review.googlesource.com/1136909
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575538}
4 files changed