[trooper-o-matic]: expose alerts feed to cross-origin requesters.
[trooper-o-matic]: expose alerts feed to cross-origin requesters.

BUG=399732

Review URL: https://codereview.chromium.org/488923002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra