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

ash/wm/window_positioner.cc

Index: ash/wm/window_positioner.cc
diff --git a/ash/wm/window_positioner.cc b/ash/wm/window_positioner.cc
index 6f93fdc6f980517f5af402d8499df11ee533f2ef..26dd0cbf0154974066e5e51877c0e75d08f35955 100644
--- a/ash/wm/window_positioner.cc
+++ b/ash/wm/window_positioner.cc
@@ -138,7 +138,7 @@ void AutoPlaceSingleWindow(aura::Window* window, bool animated) {
 }
 
 // Get the first open (non minimized) window which is on the screen defined.
-aura::Window* GetReferenceWindow(const aura::RootWindow* root_window,
+aura::Window* GetReferenceWindow(const aura::Window* root_window,
                                  const aura::Window* exclude,
                                  bool *single_window) {
   if (single_window)
@@ -212,7 +212,7 @@ void WindowPositioner::GetBoundsAndShowStateForNewWindow(
     ui::WindowShowState* show_state_out) {
 
   // Always open new window in the target display.
-  aura::RootWindow* target = Shell::GetTargetRootWindow();
+  aura::Window* target = Shell::GetTargetRootWindow();
 
   aura::Window* top_window = GetReferenceWindow(target, NULL, NULL);
   // Our window should not have any impact if we are already on top.