Support creation of toplevel mus::Windows on separate displays

ash/mus/test/wm_test_base.h

Index: ash/mus/test/wm_test_base.h
diff --git a/ash/mus/test/wm_test_base.h b/ash/mus/test/wm_test_base.h
index 8c17d39f29eff12eae1f9b1ad7ff7056c9f8e89e..1aa21b169837e4e6b8a4d0517c5b7bab3965634c 100644
--- a/ash/mus/test/wm_test_base.h
+++ b/ash/mus/test/wm_test_base.h
@@ -10,6 +10,7 @@
 
 #include "base/macros.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "ui/display/display.h"
 #include "ui/wm/public/window_types.h"
 
 namespace display {
@@ -58,7 +59,11 @@ class WmTestBase : public testing::Test {
   ui::Window* CreateTestWindow(const gfx::Rect& bounds);
   ui::Window* CreateTestWindow(const gfx::Rect& bounds,
                                ui::wm::WindowType window_type);
-  ui::Window* CreateFullscreenTestWindow();
+
+  // Creates a visibile fullscreen top level window on the display corresponding
+  // to the display_id provided.
+  ui::Window* CreateFullscreenTestWindow(
+      int64_t display_id = display::kInvalidDisplayId);
 
   // Creates a window parented to |parent|. The returned window is visible.
   ui::Window* CreateChildTestWindow(ui::Window* parent,