bluetooth: Add DeviceChanged logging in Device service.

device/bluetooth/public/interfaces/device.mojom

 // 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.
 
 module bluetooth.mojom;
 
+// TODO(crbug.com/657632): Remove when numerical values can be optional.
+struct RSSIWrapper {
+  int8 value;
+};
+
 struct DeviceInfo {
   string? name;
   string name_for_display;
   string address;
+  RSSIWrapper? rssi;
 };
 
 interface Device {
   // Gets basic information about the device. Returns null, if no device is
   // available.
   GetInfo() => (DeviceInfo? info);
 };