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

components/webauth/BUILD.gn

Index: components/webauth/BUILD.gn
diff --git a/components/webauth/BUILD.gn b/components/webauth/BUILD.gn
deleted file mode 100644
index 80da9afb1b310f04d338c8860971810f173bcacf..0000000000000000000000000000000000000000
--- a/components/webauth/BUILD.gn
+++ /dev/null
@@ -1,15 +0,0 @@
-# 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") {
-  sources = [
-    "authenticator.mojom",
-  ]
-
-  public_deps = [
-    "//mojo/common:common_custom_types",
-  ]
-}