Clear-Site-Data header support

Hello WebAppSec Team,

Hoping to clarify browser support for Clear-Site-Data header.  As of this
writing here's what I believe to be true.

Chrome
- support: as of Chrome 61
- source: https://www.chromestatus.com/feature/4713262029471744

Firefox
- support: as of Firefox 63 (shipping in October 2018)
- source: Dan Veditz' email on this distro on July 20, 2018 (thx Dan!)

Safari
- support: "No public signals"; didn't find anything definitive from the
Safari team
- sources: https://www.chromestatus.com/feature/4713262029471744 and
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Clear-Site-Data#Browser_compatibility

Edge
- support: "Public support", though I didn't come across anything
definitive from the Edge team
- sources: https://www.chromestatus.com/feature/4713262029471744 and
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Clear-Site-Data#Browser_compatibility

Would particularly appreciate any info from the Safari and Edge folks on
the list here.

Much appreciated,
Caleb Queern