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

ash/test/ash_test_base.cc

Index: ash/test/ash_test_base.cc
diff --git a/ash/test/ash_test_base.cc b/ash/test/ash_test_base.cc
index 93d770f601023725a854cfa1db020eba5c5ce8b4..6805926f6c0633a678ec1b3375270278dcb9d080 100644
--- a/ash/test/ash_test_base.cc
+++ b/ash/test/ash_test_base.cc
@@ -61,7 +61,7 @@ class AshEventGeneratorDelegate
   // aura::test::EventGeneratorDelegateAura overrides:
   aura::WindowTreeHost* GetHostAt(
       const gfx::Point& point_in_screen) const override {
-    gfx::Screen* screen = Shell::GetScreen();
+    gfx::Screen* screen = gfx::Screen::GetScreen();
     gfx::Display display = screen->GetDisplayNearestPoint(point_in_screen);
     return Shell::GetInstance()
         ->window_tree_host_manager()
@@ -244,8 +244,7 @@ aura::Window* AshTestBase::CreateTestWindowInShellWithDelegateAndType(
   if (bounds.IsEmpty()) {
     ParentWindowInPrimaryRootWindow(window);
   } else {
-    gfx::Display display =
-        Shell::GetScreen()->GetDisplayMatching(bounds);
+    gfx::Display display = gfx::Screen::GetScreen()->GetDisplayMatching(bounds);
     aura::Window* root = ash::Shell::GetInstance()
                              ->window_tree_host_manager()
                              ->GetRootWindowForDisplayId(display.id());