Add WebAuthn bindings and client interface.

third_party/WebKit/Source/modules/webauth/ScopedCredential.idl

Index: third_party/WebKit/Source/modules/webauth/ScopedCredential.idl
diff --git a/third_party/WebKit/Source/modules/webauth/ScopedCredential.idl b/third_party/WebKit/Source/modules/webauth/ScopedCredential.idl
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+++ b/third_party/WebKit/Source/modules/webauth/ScopedCredential.idl
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+// 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/#enumdef-scopedcredentialtype
+
+enum ScopedCredentialType {
+    "ScopedCred"
+};
+
+// https://w3c.github.io/webauthn/#scopedcredential
+
+[
+    RuntimeEnabled=WebAuth,
+    SecureContext
+] interface ScopedCredential {
+    readonly attribute ScopedCredentialType type;
+    readonly attribute ArrayBuffer id;
+};