Add a basic Mojo SerialService that implements GetDevices().

device/serial/serial.mojom

 // Copyright 2014 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 device.serial {
 
 struct DeviceInfo {
   string path;
   uint16 vendor_id;
   bool has_vendor_id = false;
@@ skipping 61 matching lines @@
   bool has_rts = false;
 };
 
 struct DeviceControlSignals {
   bool dcd;
   bool cts;
   bool ri;
   bool dsr;
 };
 
+interface SerialService {
+  GetDevices() => (DeviceInfo[] devices);
+};
+
 }