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..f7014b0b7dd01889201db049a8309566419574d6 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 +61,9 @@ 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,
@@ -71,7 +76,6 @@ class WmTestBase : public testing::Test {
 
  private:
   friend class AshTestImplMus;
-
   bool setup_called_ = false;
   bool teardown_called_ = false;
   std::unique_ptr<WmTestHelper> test_helper_;