Add a switch for tablet power button behavior

ash/wm/power_button_controller.cc

 // Copyright (c) 2012 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/wm/power_button_controller.h"
 
 #include "ash/common/accelerators/accelerator_controller.h"
 #include "ash/common/ash_switches.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/system/tray/system_tray.h"
@@ skipping 26 matching lines @@
 #endif
       brightness_is_zero_(false),
       internal_display_off_and_external_display_on_(false),
       has_legacy_power_button_(
           base::CommandLine::ForCurrentProcess()->HasSwitch(
               switches::kAuraLegacyPowerButton)),
       lock_state_controller_(controller) {
 #if defined(OS_CHROMEOS)
   chromeos::DBusThreadManager::Get()->GetPowerManagerClient()->AddObserver(
       this);
-  tablet_controller_.reset(
-      new TabletPowerButtonController(lock_state_controller_));
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kAshEnableTabletPowerButton)) {
+    tablet_controller_.reset(
+        new TabletPowerButtonController(lock_state_controller_));
+  }
   Shell::GetInstance()->display_configurator()->AddObserver(this);
 #endif
   Shell::GetInstance()->PrependPreTargetHandler(this);
 }
 
 PowerButtonController::~PowerButtonController() {
   Shell::GetInstance()->RemovePreTargetHandler(this);
 #if defined(OS_CHROMEOS)
   Shell::GetInstance()->display_configurator()->RemoveObserver(this);
   tablet_controller_.reset();
@@ skipping 14 matching lines @@
   if (lock_state_controller_->ShutdownRequested())
     return;
 
 #if defined(OS_CHROMEOS)
   bool should_take_screenshot = down && volume_down_pressed_ &&
                                 WmShell::Get()
                                     ->maximize_mode_controller()
                                     ->IsMaximizeModeWindowManagerEnabled();
 
   if (!has_legacy_power_button_ && !should_take_screenshot &&
+      tablet_controller_ &&
       tablet_controller_->ShouldHandlePowerButtonEvents()) {
     tablet_controller_->OnPowerButtonEvent(down, timestamp);
     return;
   }
 #endif
 
   // Avoid starting the lock/shutdown sequence if the power button is pressed
   // while the screen is off (http://crbug.com/128451), unless an external
   // display is still on (http://crosbug.com/p/24912).
   if (brightness_is_zero_ && !internal_display_off_and_external_display_on_)
@@ skipping 111 matching lines @@
 }
 
 void PowerButtonController::PowerButtonEventReceived(
     bool down,
     const base::TimeTicks& timestamp) {
   OnPowerButtonEvent(down, timestamp);
 }
 #endif  // defined(OS_CHROMEOS)
 
 }  // namespace ash