Add WebAuthn bindings and client interface.

third_party/WebKit/Source/modules/webauthentication/WebAuthentication.idl

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// https://w3c.github.io/webauthn/#webauthentication
+
+[
+    RuntimeEnabled=WebAuthentication,
+    SecureContext
+] interface WebAuthentication {
+    [CallWith=ScriptState] Promise<sequence<ScopedCredentialInfo>> makeCredential(Account accountInformation, sequence<ScopedCredentialParameters> cryptoParameters, BufferSource attestationChallenge, optional ScopedCredentialOptions options);
+    [CallWith=ScriptState] Promise<sequence<WebAuthnAssertion>> getAssertion(BufferSource assertionChallenge, optional AssertionOptions options);
+};