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..28c6720ba287af5b133c0c1be7ff9a28dd8fd3f0 100644
--- a/ash/mus/test/wm_test_base.h
+++ b/ash/mus/test/wm_test_base.h
@@ -51,6 +51,9 @@ class WmTestBase : public testing::Test {
   display::Display GetPrimaryDisplay();
   display::Display GetSecondaryDisplay();
 
+  RootWindowController* GetPrimaryRootWindowController();
+  RootWindowController* GetSecondaryRootWindowController();
+
   // Creates a top level window visible window in the appropriate container.
   // NOTE: you can explicitly destroy the returned value if necessary, but it
   // will also be automatically destroyed when the WindowTreeClient is
@@ -58,7 +61,8 @@ 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();
+
+  ui::Window* CreateFullscreenTestWindow(RootWindowController* root);
 
   // Creates a window parented to |parent|. The returned window is visible.
   ui::Window* CreateChildTestWindow(ui::Window* parent,