Moves maintaining ShellObservers back to Shell

ash/common/wm/maximize_mode/maximize_mode_window_manager.cc

Index: ash/common/wm/maximize_mode/maximize_mode_window_manager.cc
diff --git a/ash/common/wm/maximize_mode/maximize_mode_window_manager.cc b/ash/common/wm/maximize_mode/maximize_mode_window_manager.cc
index 9cafa32700c574e80a0572a47fd0398e8f85a2be..003fd899b6204b3ba7b4d8ed28192c00e470831b 100644
--- a/ash/common/wm/maximize_mode/maximize_mode_window_manager.cc
+++ b/ash/common/wm/maximize_mode/maximize_mode_window_manager.cc
@@ -48,7 +48,7 @@ MaximizeModeWindowManager::~MaximizeModeWindowManager() {
   for (aura::Window* window : added_windows_)
     window->RemoveObserver(this);
   added_windows_.clear();
-  WmShell::Get()->RemoveShellObserver(this);
+  Shell::GetInstance()->RemoveShellObserver(this);
   display::Screen::GetScreen()->RemoveObserver(this);
   EnableBackdropBehindTopWindowOnEachDisplay(false);
   RemoveWindowCreationObservers();
@@ -206,7 +206,7 @@ MaximizeModeWindowManager::MaximizeModeWindowManager()
   AddWindowCreationObservers();
   EnableBackdropBehindTopWindowOnEachDisplay(true);
   display::Screen::GetScreen()->AddObserver(this);
-  WmShell::Get()->AddShellObserver(this);
+  Shell::GetInstance()->AddShellObserver(this);
   event_handler_ = WmShell::Get()->CreateMaximizeModeEventHandler();
 }