chromeos: converts TopLevelWindowFactoryTest to AshTestBase

ash/display/window_tree_host_manager_unittest.cc

Index: ash/display/window_tree_host_manager_unittest.cc
diff --git a/ash/display/window_tree_host_manager_unittest.cc b/ash/display/window_tree_host_manager_unittest.cc
index 6f03fff6a006a85327fe9af913a7beb1b3e40b41..8af27cfcb9b08ae35e06a6b6e7ee77f5d0da4e91 100644
--- a/ash/display/window_tree_host_manager_unittest.cc
+++ b/ash/display/window_tree_host_manager_unittest.cc
@@ -180,11 +180,6 @@ class TestObserver : public WindowTreeHostManager::Observer,
   DISALLOW_COPY_AND_ASSIGN(TestObserver);
 };
 
-display::Display GetPrimaryDisplay() {
-  return display::Screen::GetScreen()->GetDisplayNearestWindow(
-      Shell::GetAllRootWindows()[0]);
-}
-
 class TestHelper {
  public:
   explicit TestHelper(test::AshTestBase* delegate);