Re-enable calls on |display_manager_| for Windows in DisplayManagerTestApi

ash/wm/system_modal_container_layout_manager_unittest.cc

Index: ash/wm/system_modal_container_layout_manager_unittest.cc
diff --git a/ash/wm/system_modal_container_layout_manager_unittest.cc b/ash/wm/system_modal_container_layout_manager_unittest.cc
index f3e35249254f07f79190f1f761ca02631fb93f75..dfb17eccfc4a4f12aeea7c84902fdfd9ecd7c0a3 100644
--- a/ash/wm/system_modal_container_layout_manager_unittest.cc
+++ b/ash/wm/system_modal_container_layout_manager_unittest.cc
@@ -770,6 +770,9 @@ TEST_F(SystemModalContainerLayoutManagerTest, BlockAllEvents) {
 
 // Make sure that events are properly blocked in multi displays environment.
 TEST_F(SystemModalContainerLayoutManagerTest, BlockEventsInMultiDisplays) {
+  if (!SupportsMultipleDisplays())
+    return;
+
   UpdateDisplay("500x500, 500x500");
   InputTestDelegate delegate;
   delegate.RunTest(this);