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

ash/shelf/shelf_window_watcher.cc

Index: ash/shelf/shelf_window_watcher.cc
diff --git a/ash/shelf/shelf_window_watcher.cc b/ash/shelf/shelf_window_watcher.cc
index 4146b50952554cd8c746eb7090f3d9f46e4dd469..80df891dd40b2c37103aaa8f2cb53625fcb0c61e 100644
--- a/ash/shelf/shelf_window_watcher.cc
+++ b/ash/shelf/shelf_window_watcher.cc
@@ -120,11 +120,11 @@ ShelfWindowWatcher::ShelfWindowWatcher(
        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) {