[WebAuthn](https://w3c.github.io/webauthn/) builds upon the Credential
Management API and provides for origin-scoped, public-key credentials.
It replaces the FIDO U2F API which is currently used to implement this.
While the webauthn spec is still only a candidate recommendation, the
types in this change are sufficient to exercise basic functionality with
Firefox Nightly. (Note: that requires changes in about:config to enable
this API.)
Removes the copied (now out of date) definition of RequestInit.
Once 2.3 is "stable" for DefinitelyTyped (early June),
CMRequestInit will be updated to match the latest lib.dom.d.ts
definition.
