Replace WmWindowObserver with aura::WindowObserver.

ash/shelf/shelf_window_targeter.cc

Index: ash/shelf/shelf_window_targeter.cc
diff --git a/ash/shelf/shelf_window_targeter.cc b/ash/shelf/shelf_window_targeter.cc
index e8198b2dd782a86b2cc5a53949f8c53f5915ba1c..24ba0e8bb5dba3b2e8d589201bdfc22527a84989 100644
--- a/ash/shelf/shelf_window_targeter.cc
+++ b/ash/shelf/shelf_window_targeter.cc
@@ -8,6 +8,7 @@
 #include "ash/common/shelf/wm_shelf.h"
 #include "ash/common/wm_window.h"
 #include "ash/public/cpp/shelf_types.h"
+#include "ui/aura/window.h"
 
 namespace ash {
 
@@ -29,7 +30,7 @@ ShelfWindowTargeter::ShelfWindowTargeter(WmWindow* container, WmShelf* shelf)
                                      gfx::Insets()),
       shelf_(shelf) {
   WillChangeVisibilityState(shelf_->GetVisibilityState());
-  container->AddObserver(this);
+  container->aura_window()->AddObserver(this);
   shelf_->AddObserver(this);
 }
 
@@ -38,7 +39,7 @@ ShelfWindowTargeter::~ShelfWindowTargeter() {
   DCHECK(!shelf_);
 }
 
-void ShelfWindowTargeter::OnWindowDestroying(WmWindow* window) {
+void ShelfWindowTargeter::OnWindowDestroying(aura::Window* window) {
   window->RemoveObserver(this);
   shelf_->RemoveObserver(this);
   shelf_ = nullptr;