Separate ash::test::DisplayManagerTestApi from ash

ash/display/display_manager.cc

Index: ash/display/display_manager.cc
diff --git a/ash/display/display_manager.cc b/ash/display/display_manager.cc
index fa3f302f04bff0b7a94a0c6b2c7bb70d09ae7da1..cbae3faa478851ee24e0af3023f69a0936455727 100644
--- a/ash/display/display_manager.cc
+++ b/ash/display/display_manager.cc
@@ -1433,4 +1433,12 @@ void DisplayManager::RemoveObserver(display::DisplayObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
+const display::Display& DisplayManager::GetSecondaryDisplay() const {
+  CHECK_LE(2U, GetNumDisplays());
+  return GetDisplayAt(0).id() ==
+                 display::Screen::GetScreen()->GetPrimaryDisplay().id()
+             ? GetDisplayAt(1)
+             : GetDisplayAt(0);
+}
+
 }  // namespace ash