[DeviceService] Add end-to-end browsertest for PowerMonitor

content/shell/common/power_monitor_test.mojom

+// 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.
+
+module content.mojom;
+
+// This interface is only for the sake of browser test to query current power
+// state observed in remote child processes.
+interface PowerMonitorTest {
+  // The power state is reported immediatelly if there is already a change
+  // available to notify, otherwise wait until a change happen. Overlapping
+  // calls to QueryNextState are prohibited.
+  QueryNextState() => (bool on_battery_power);
+};