[webauthn] Need to clearly define what it means to SHA-256 hash the "host" output of the document.domain setter

bzbarsky has just created a new issue for 
https://github.com/w3c/webauthn:

== Need to clearly define what it means to SHA-256 hash the "host" 
output of the document.domain setter ==
The output is a Unicode string.  SHA-256 takes bytes as input.  The 
exact conversion of Unicode to bytes here needs to be defined.

This can be sort of guessed at, because I _think_ the actual string 
involved will always be in the ASCII range.  But I'm not 100% sure, 
and it would be best to eliminate the guessing.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/258 using your GitHub account