Replace WmWindowObserver with aura::WindowObserver.

ash/common/wm/window_dimmer.h

Index: ash/common/wm/window_dimmer.h
diff --git a/ash/common/wm/window_dimmer.h b/ash/common/wm/window_dimmer.h
index 5250575518a451fc52a5725ff150ad25aaeb394e..48ded9775d9393c43dd52d45331a2f8e77aa1598 100644
--- a/ash/common/wm/window_dimmer.h
+++ b/ash/common/wm/window_dimmer.h
@@ -6,11 +6,13 @@
 #define ASH_COMMON_WINDOW_DIMMER_H_
 
 #include "ash/ash_export.h"
-#include "ash/common/wm_window_observer.h"
 #include "base/macros.h"
+#include "ui/aura/window_observer.h"
 
 namespace ash {
 
+class WmWindow;
+
 // WindowDimmer creates a window whose opacity is animated by way of
 // SetDimOpacity() and whose size matches that of its parent. WindowDimmer is
 // intended to be used in cases where a certain set of windows need to appear
@@ -20,7 +22,7 @@ namespace ash {
 // deleted if WindowDimmer is deleted. It is expected that WindowDimmer is
 // deleted when the parent window is deleted (such as happens with
 // WmWindowUserData).
-class ASH_EXPORT WindowDimmer : public WmWindowObserver {
+class ASH_EXPORT WindowDimmer : public aura::WindowObserver {
  public:
   // Creates a new WindowDimmer. The window() created by WindowDimmer is added
   // to |parent| and stacked above all other child windows.
@@ -33,13 +35,12 @@ class ASH_EXPORT WindowDimmer : public WmWindowObserver {
   WmWindow* window() { return window_; }
 
   // NOTE: WindowDimmer is an observer for both |parent_| and |window_|.
-  // WmWindowObserver:
-  void OnWindowBoundsChanged(WmWindow* window,
+  // aura::WindowObserver:
+  void OnWindowBoundsChanged(aura::Window* window,
                              const gfx::Rect& old_bounds,
                              const gfx::Rect& new_bounds) override;
-  void OnWindowDestroying(WmWindow* window) override;
-  void OnWindowTreeChanging(WmWindow* window,
-                            const TreeChangeParams& params) override;
+  void OnWindowDestroying(aura::Window* window) override;
+  void OnWindowHierarchyChanging(const HierarchyChangeParams& params) override;
 
  private:
   WmWindow* parent_;