Add ArcAuthService to handle IPC request from ARC

components/arc/common/auth.mojom

Index: components/arc/common/auth.mojom
diff --git a/components/arc/common/auth.mojom b/components/arc/common/auth.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..6bf979255d52c6e762d4eb4954c39a39ebd658cb
--- /dev/null
+++ b/components/arc/common/auth.mojom
@@ -0,0 +1,15 @@
+// Copyright 2015 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.
+
+module arc;
+
+interface AuthHost {
+  // Returns an authorization code, which can be used to sign in.
+  GetAuthCode() => (string auth_code);
+};
+
+interface AuthInstance {
+  // Establishes full-duplex communication with the host.
+  Init(AuthHost host_ptr);
+};