Remove now-unnecessary wrappers around gfx::Screen::GetScreen()

ash/shelf/shelf_window_watcher.cc

 // Copyright 2013 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/shelf/shelf_window_watcher.h"
 
 #include <utility>
 
 #include "ash/display/window_tree_host_manager.h"
 #include "ash/shelf/shelf_constants.h"
@@ skipping 102 matching lines @@
       observed_root_windows_(&root_window_observer_),
       observed_removed_windows_(&removed_window_observer_),
       observed_activation_clients_(this) {
   // We can't assume all RootWindows have the same ActivationClient.
   // Add a RootWindow and its ActivationClient to the observed list.
   aura::Window::Windows root_windows = Shell::GetAllRootWindows();
   for (aura::Window::Windows::const_iterator it = root_windows.begin();
        it != root_windows.end(); ++it)
     OnRootWindowAdded(*it);
 
-  Shell::GetScreen()->AddObserver(this);
+  gfx::Screen::GetScreen()->AddObserver(this);
 }
 
 ShelfWindowWatcher::~ShelfWindowWatcher() {
-  Shell::GetScreen()->RemoveObserver(this);
+  gfx::Screen::GetScreen()->RemoveObserver(this);
 }
 
 void ShelfWindowWatcher::AddShelfItem(aura::Window* window) {
   const ShelfItemDetails* item_details =
       GetShelfItemDetailsForWindow(window);
   ShelfItem item;
   ShelfID id = model_->next_id();
   item.status = wm::IsActiveWindow(window) ? STATUS_ACTIVE: STATUS_RUNNING;
   SetShelfItemDetailsForShelfItem(&item, *item_details);
   SetShelfIDForWindow(id, window);
@@ skipping 147 matching lines @@
   // Instead, we remove an observer from RootWindow and ActivationClient in the
   // OnRootWindowDestroyed().
   // Do nothing here.
 }
 
 void ShelfWindowWatcher::OnDisplayMetricsChanged(const gfx::Display&,
                                                  uint32_t) {
 }
 
 }  // namespace ash