[beacon-tests] Support referrer policy in sendBeacon (#5357)

MozReview-Commit-ID: FEyqInOkiT6

Upstreamed from https://bugzilla.mozilla.org/show_bug.cgi?id=1192940 [ci skip]

View on GitHub: https://github.com/w3c/web-platform-tests/pull/5357

Received on Tuesday, 4 April 2017 15:56:39 UTC