Add TODO to rename to FeaturePolicy::Allowlist

Add a TODO linking to the bug as a reminder, in case the rename makes
sense along with other changes.

Also updated all the comments in the file to use "allowlist".

Bug: 822317
Change-Id: I9ac55077491be776a50cadc723bf3cac41c8ebaf
Reviewed-on: https://chromium-review.googlesource.com/964613
Commit-Queue: Jason Chase <chasej@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543962}
1 file changed