Moves maintaining ShellObservers back to Shell

ash/system/chromeos/power/tablet_power_button_controller.cc

Index: ash/system/chromeos/power/tablet_power_button_controller.cc
diff --git a/ash/system/chromeos/power/tablet_power_button_controller.cc b/ash/system/chromeos/power/tablet_power_button_controller.cc
index 672e54c199841b6ecf569733fd6ad71c0a2b1baa..519962461c38c227ed4bb1bc6873bc4f6a099ad6 100644
--- a/ash/system/chromeos/power/tablet_power_button_controller.cc
+++ b/ash/system/chromeos/power/tablet_power_button_controller.cc
@@ -82,7 +82,7 @@ TabletPowerButtonController::TabletPowerButtonController(
       weak_ptr_factory_(this) {
   chromeos::DBusThreadManager::Get()->GetPowerManagerClient()->AddObserver(
       this);
-  WmShell::Get()->AddShellObserver(this);
+  Shell::GetInstance()->AddShellObserver(this);
   // TODO(mash): Provide a way for this class to observe stylus events:
   // http://crbug.com/682460
   if (ui::InputDeviceManager::HasInstance())
@@ -96,7 +96,7 @@ TabletPowerButtonController::~TabletPowerButtonController() {
   Shell::GetInstance()->RemovePreTargetHandler(this);
   if (ui::InputDeviceManager::HasInstance())
     ui::InputDeviceManager::GetInstance()->RemoveObserver(this);
-  WmShell::Get()->RemoveShellObserver(this);
+  Shell::GetInstance()->RemoveShellObserver(this);
   chromeos::DBusThreadManager::Get()->GetPowerManagerClient()->RemoveObserver(
       this);
 }