[webauthn] new commits pushed by vijaybh

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

* fixup getAssertion, polish algorithms (#371)

* make getAssertion() more precise a la makeCredential

* fixes per vijaybh's review.

* 'was specd'=>'present/not present' fixes #251. + markup canonicalization fixups

* fixes some of the prior 'fixes' (haste made waste)

* fixes #267 wrt (not) empty lists

* fix #271

* fix another 'empty' thing

* update acks

* attempt resolution of #277

* fix getA() cutnpaste: ScopedCredentialOptions => AssertionOptions

* attempts improvement of #280

* cleanup fixes for #254 and #271

* fixing relevant origin bugs, thx vijaybh!

* polish token binding occurances

* apply global and task src to getA() fixes #277
  by =JeffH
https://github.com/w3c/webauthn/commit/985f88e1effa1662117d88fd0f9f6eb09a4512c5