[webauthn] new commits pushed by vijaybh

The following commits were just pushed by vijaybh to 
https://github.com/w3c/webauthn:

* IDL cleanup: don't require callers to create Credential objects

This is the minimal version of the change required to fix the IDL 
issue
that @jcjones brought up regarding Credential objects. Unlike PR #158,
it retains the credential type and leaves the actual API surface and
samples unchanged.

If we pick this approach, PR #158 can be closed out and will no longer
be necessary.
  by Vijay Bharadwaj
https://github.com/w3c/webauthn/commit/c79e21b6ad5804e84bbee4983706366455000494