Make GetRootWindow() return a Window instead of a RootWindow.

ash/display/mirror_window_controller_unittest.cc

Index: ash/display/mirror_window_controller_unittest.cc
diff --git a/ash/display/mirror_window_controller_unittest.cc b/ash/display/mirror_window_controller_unittest.cc
index fd3cc818e02aa926c64323cd0ee07dafee711618..e53f0bad5ed7328f90afb03ef2630e51c68b1948 100644
--- a/ash/display/mirror_window_controller_unittest.cc
+++ b/ash/display/mirror_window_controller_unittest.cc
@@ -75,7 +75,7 @@ TEST_F(MirrorWindowControllerTest, MAYBE_MirrorCursorBasic) {
   DisplayManager* display_manager = Shell::GetInstance()->display_manager();
   display_manager->SetSoftwareMirroring(true);
   UpdateDisplay("400x400,400x400");
-  aura::RootWindow* root = Shell::GetInstance()->GetPrimaryRootWindow();
+  aura::Window* root = Shell::GetInstance()->GetPrimaryRootWindow();
   scoped_ptr<aura::Window> window(aura::test::CreateTestWindowWithDelegate(
       &test_window_delegate,
       0,
@@ -132,7 +132,7 @@ TEST_F(MirrorWindowControllerTest, MAYBE_MirrorCursorRotate) {
   DisplayManager* display_manager = Shell::GetInstance()->display_manager();
   display_manager->SetSoftwareMirroring(true);
   UpdateDisplay("400x400,400x400");
-  aura::RootWindow* root = Shell::GetInstance()->GetPrimaryRootWindow();
+  aura::Window* root = Shell::GetInstance()->GetPrimaryRootWindow();
   scoped_ptr<aura::Window> window(aura::test::CreateTestWindowWithDelegate(
       &test_window_delegate,
       0,
@@ -199,7 +199,7 @@ TEST_F(MirrorWindowControllerTest, MAYBE_MirrorCursorLocations) {
   // Test with device scale factor.
   UpdateDisplay("400x600*2,400x600");
 
-  aura::RootWindow* root = Shell::GetInstance()->GetPrimaryRootWindow();
+  aura::Window* root = Shell::GetInstance()->GetPrimaryRootWindow();
   aura::test::EventGenerator generator(root);
   generator.MoveMouseToInHost(10, 20);