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

ash/wm/coordinate_conversion.h

Index: ash/wm/coordinate_conversion.h
diff --git a/ash/wm/coordinate_conversion.h b/ash/wm/coordinate_conversion.h
index b1ee83cd200176c6674b79cd4cafa6c3a1149037..f5c1af8d20f8d3af47c570591fec53dcf239a651 100644
--- a/ash/wm/coordinate_conversion.h
+++ b/ash/wm/coordinate_conversion.h
@@ -8,7 +8,6 @@
 #include "ash/ash_export.h"
 
 namespace aura {
-class RootWindow;
 class Window;
 }  // namespace gfx
 
@@ -23,11 +22,11 @@ namespace wm {
 // Returns the RootWindow at |point| in the virtual screen coordinates.
 // Returns NULL if the root window does not exist at the given
 // point.
-ASH_EXPORT aura::RootWindow* GetRootWindowAt(const gfx::Point& point);
+ASH_EXPORT aura::Window* GetRootWindowAt(const gfx::Point& point);
 
 // Returns the RootWindow that shares the most area with |rect| in
 // the virtual scren coordinates.
-ASH_EXPORT aura::RootWindow* GetRootWindowMatching(const gfx::Rect& rect);
+ASH_EXPORT aura::Window* GetRootWindowMatching(const gfx::Rect& rect);
 
 // Converts the |point| from a given |window|'s coordinates into the screen
 // coordinates.