Add a switch for tablet power button behavior

ash/system/chromeos/power/tablet_power_button_controller_unittest.cc

 // 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.
 
 #include "ash/system/chromeos/power/tablet_power_button_controller.h"
 
 #include <memory>
 
 #include "ash/common/ash_switches.h"
 #include "ash/common/session/session_state_delegate.h"
@@ skipping 32 matching lines @@
   ~TabletPowerButtonControllerTest() override {}
 
   void SetUp() override {
     // This also initializes DBusThreadManager.
     std::unique_ptr<chromeos::DBusThreadManagerSetter> dbus_setter =
         chromeos::DBusThreadManager::GetSetterForTesting();
     power_manager_client_ = new chromeos::FakePowerManagerClient();
     dbus_setter->SetPowerManagerClient(base::WrapUnique(power_manager_client_));
     base::CommandLine::ForCurrentProcess()->AppendSwitch(
         switches::kAshEnableTouchViewTesting);
+    base::CommandLine::ForCurrentProcess()->AppendSwitch(
+        switches::kAshEnableTabletPowerButton);
     AshTestBase::SetUp();
 
     lock_state_controller_ = Shell::GetInstance()->lock_state_controller();
     tablet_controller_ = Shell::GetInstance()
                              ->power_button_controller()
                              ->tablet_power_button_controller_for_test();
     test_api_ = base::MakeUnique<TabletPowerButtonController::TestApi>(
         tablet_controller_);
     generator_ = &AshTestBase::GetEventGenerator();
     power_manager_client_->SendBrightnessChanged(kNonZeroBrightness, false);
@@ skipping 234 matching lines @@
   EXPECT_TRUE(GetBacklightsForcedOff());
 
   generator_->EnterPenPointerMode();
   generator_->MoveMouseBy(1, 1);
   EXPECT_TRUE(GetBacklightsForcedOff());
   generator_->ExitPenPointerMode();
 }
 
 }  // namespace test
 }  // namespace ash