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

ash/wm/workspace/phantom_window_controller.h

Index: ash/wm/workspace/phantom_window_controller.h
diff --git a/ash/wm/workspace/phantom_window_controller.h b/ash/wm/workspace/phantom_window_controller.h
index 4a9279256bf3a61c1d6df18ff30f4cf76c895798..77d4a2860353a0393c0d703f8c09187f4c3788ba 100644
--- a/ash/wm/workspace/phantom_window_controller.h
+++ b/ash/wm/workspace/phantom_window_controller.h
@@ -13,7 +13,6 @@
 #include "ui/gfx/rect.h"
 
 namespace aura {
-class RootWindow;
 class Window;
 }
 
@@ -65,7 +64,7 @@ class ASH_EXPORT PhantomWindowController : public gfx::AnimationDelegate {
 
   // Creates, shows and returns a phantom widget at |bounds|
   // with kShellWindowId_ShelfContainer in |root_window| as a parent.
-  views::Widget* CreatePhantomWidget(aura::RootWindow* root_window,
+  views::Widget* CreatePhantomWidget(aura::Window* root_window,
                                      const gfx::Rect& bounds_in_screen);
 
   // Window the phantom is placed beneath.