[w3c/webauthn] adbda3: Fix feature detection. (#808)

  Branch: refs/heads/master
  Home:   https://github.com/w3c/webauthn
  Commit: adbda3ec2ba2cfce06f57a7da45b1fca5170fbbb
      https://github.com/w3c/webauthn/commit/adbda3ec2ba2cfce06f57a7da45b1fca5170fbbb
  Author: Mike West <mike@mikewest.org>
  Date:   2018-02-21 (Wed, 21 Feb 2018)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Fix feature detection. (#808)

The current feature detection code will throw a 'ReferenceError', as 'PublicKeyCredential'
doesn't exist. Adding 'window.' turns it into a property lookup, which fails gracefully.