Adds WmWindow and converts WindowState to use it

ash/wm/common/wm_window_observer.h

Index: ash/wm/common/wm_window_observer.h
diff --git a/ash/wm/common/wm_window_observer.h b/ash/wm/common/wm_window_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..97500c0e4bdb8a6af6bc1b36044f80b16c96fbf2
--- /dev/null
+++ b/ash/wm/common/wm_window_observer.h
@@ -0,0 +1,52 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ASH_WM_COMMON_WM_WINDOW_OBSERVER_H_
+#define ASH_WM_COMMON_WM_WINDOW_OBSERVER_H_
+
+#include "ash/ash_export.h"
+
+namespace gfx {
+class Rect;
+}
+
+namespace ash {
+namespace wm {
+
+class WmWindow;
+enum class WmWindowProperty;
+
+class ASH_EXPORT WmWindowObserver {
+ public:
+  struct TreeChangeParams {
+    WmWindow* target = nullptr;
+    WmWindow* old_parent = nullptr;
+    WmWindow* new_parent = nullptr;
+  };
+
+  // |window| is the window the observer was added to, which is not necessarily
+  // the window that was added/removed.
+  virtual void OnWindowTreeChanged(WmWindow* window,
+                                   const TreeChangeParams& params) {}
+
+  virtual void OnWindowPropertyChanged(WmWindow* window,
+                                       WmWindowProperty property,
+                                       intptr_t old) {}
+
+  virtual void OnWindowStackingChanged(WmWindow* window) {}
+
+  virtual void OnWindowDestroying(WmWindow* window) {}
+
+  virtual void OnWindowBoundsChanged(WmWindow* window,
+                                     const gfx::Rect& old_bounds,
+                                     const gfx::Rect& new_bounds) {}
+
+ protected:
+  virtual ~WmWindowObserver() {}
+};
+
+}  // namespace wm
+}  // namespace ash
+
+#endif  // ASH_WM_COMMON_WM_WINDOW_OBSERVER_H_