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

ash/desktop_background/desktop_background_controller_unittest.cc

Index: ash/desktop_background/desktop_background_controller_unittest.cc
diff --git a/ash/desktop_background/desktop_background_controller_unittest.cc b/ash/desktop_background/desktop_background_controller_unittest.cc
index 4d9194979c1f4f3ecd1fdf5b9d42245395d18972..6a7d19c4887bad519613ccf77f82e11aba772a6e 100644
--- a/ash/desktop_background/desktop_background_controller_unittest.cc
+++ b/ash/desktop_background/desktop_background_controller_unittest.cc
@@ -45,7 +45,7 @@ const int kLockScreenBackgroundId =
 
 // Returns number of child windows in a shell window container.
 int ChildCountForContainer(int container_id) {
-  RootWindow* root = ash::Shell::GetPrimaryRootWindow();
+  Window* root = ash::Shell::GetPrimaryRootWindow();
   Window* container = root->GetChildById(container_id);
   return static_cast<int>(container->children().size());
 }