mash: Fix TabletPowerButtonController segfault.

ash/system/chromeos/power/tablet_power_button_controller.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 "ash/common/accessibility_delegate.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/shell_delegate.h"
 #include "ash/common/wm/maximize_mode/maximize_mode_controller.h"
@@ skipping 69 matching lines @@
 TabletPowerButtonController::TabletPowerButtonController(
     LockStateController* controller)
     : tick_clock_(new base::DefaultTickClock()),
       last_resume_time_(base::TimeTicks()),
       force_off_on_button_up_(true),
       controller_(controller),
       weak_ptr_factory_(this) {
   chromeos::DBusThreadManager::Get()->GetPowerManagerClient()->AddObserver(
       this);
   WmShell::Get()->AddShellObserver(this);
-  ui::InputDeviceManager::GetInstance()->AddObserver(this);
+  // TODO(mash): Provide a way for this class to observe stylus events:
+  // http://crbug.com/682460
+  if (ui::InputDeviceManager::HasInstance())
+    ui::InputDeviceManager::GetInstance()->AddObserver(this);
   Shell::GetInstance()->PrependPreTargetHandler(this);
 
   GetInitialBacklightsForcedOff();
 }
 
 TabletPowerButtonController::~TabletPowerButtonController() {
   Shell::GetInstance()->RemovePreTargetHandler(this);
-  ui::InputDeviceManager::GetInstance()->RemoveObserver(this);
+  if (ui::InputDeviceManager::HasInstance())
+    ui::InputDeviceManager::GetInstance()->RemoveObserver(this);
   WmShell::Get()->RemoveShellObserver(this);
   chromeos::DBusThreadManager::Get()->GetPowerManagerClient()->RemoveObserver(
       this);
 }
 
 bool TabletPowerButtonController::ShouldHandlePowerButtonEvents() const {
   return IsTabletModeSupported();
 }
 
 void TabletPowerButtonController::OnPowerButtonEvent(
@@ skipping 148 matching lines @@
         FROM_HERE, base::TimeDelta::FromMilliseconds(kLockScreenTimeoutMs),
         this, &TabletPowerButtonController::OnLockScreenTimeout);
   }
 }
 
 void TabletPowerButtonController::OnLockScreenTimeout() {
   WmShell::Get()->GetSessionStateDelegate()->LockScreen();
 }
 
 }  // namespace ash