Add WorkspaceObserver to observe changes in workspace state

ui/aura_shell/workspace/workspace_observer.h

Index: ui/aura_shell/workspace/workspace_observer.h
diff --git a/ui/aura_shell/workspace/workspace_observer.h b/ui/aura_shell/workspace/workspace_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..e0387bb1157d6ab669664c3166e2bf41ccdd8737
--- /dev/null
+++ b/ui/aura_shell/workspace/workspace_observer.h
@@ -0,0 +1,40 @@
+// Copyright (c) 2011 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 UI_AURA_SHELL_WORKSPACE_WORKSPACE_OBSERVER_H_
+#define UI_AURA_SHELL_WORKSPACE_WORKSPACE_OBSERVER_H_
+#pragma once
+
+#include "ui/aura_shell/aura_shell_export.h"
+
+namespace aura {
+class Window;
+}
+
+namespace aura_shell {
+namespace internal {
+class Workspace;
+class WorkspaceManager;
+
+// A class to observe changes in workspace state.
+class AURA_SHELL_EXPORT WorkspaceObserver {
+ public:
+  // Invoked when |start| window is moved and inserted
+  // at the |target| window's position by |WorkspaceManager::RotateWindow|.
+  virtual void WindowMoved(WorkspaceManager* manager,
+                           aura::Window* source,
+                           aura::Window* target) = 0;
+
+  // Invoked when the active workspace changes. |old| is
+  // the old active workspace and can be NULL.
+  virtual void ActiveWorkspaceChanged(WorkspaceManager* manager,
+                                      Workspace* old) = 0;
+ protected:
+  virtual ~WorkspaceObserver() {}
+};
+
+}  // namespace internal
+}  // namespace aura_shell
+
+#endif  // UI_AURA_SHELL_WORKSPACE_WORKSPACE_OBSERVER_H_