Add the Mojo implementation of authenticator.mojom's MakeCredential.

components/webauth/BUILD.gn

 # Copyright 2017 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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
-mojom("authenticator") {
+source_set("authenticator") {
+  sources = [
+    "authenticator_impl.cc",
+    "authenticator_impl.h",
+  ]
+
+  deps = [
+    ":webauth",
+    "//base",
+    "//content/public/browser",
+    "//mojo/common",
+    "//mojo/public/cpp/bindings",
+  ]
+}
+
+mojom("webauth") {
   sources = [
     "authenticator.mojom",
   ]
 
   public_deps = [
     "//mojo/common:common_custom_types",
   ]
 }