Revert of Rename wm_core_export.h to wm_export.h and the wm_core target to just wm

ui/wm/core/transient_window_observer.h

 // Copyright 2014 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_WM_CORE_TRANSIENT_WINDOW_OBSERVER_H_
 #define UI_WM_CORE_TRANSIENT_WINDOW_OBSERVER_H_
 
-#include "ui/wm/wm_export.h"
+#include "ui/wm/core/wm_core_export.h"
 
 namespace aura {
 class Window;
 }
 
 namespace wm {
 
-class WM_EXPORT TransientWindowObserver {
+class WM_CORE_EXPORT TransientWindowObserver {
  public:
   // Called when a transient child is added to |window|.
   virtual void OnTransientChildAdded(aura::Window* window,
                                      aura::Window* transient) = 0;
 
   // Called when a transient child is removed from |window|.
   virtual void OnTransientChildRemoved(aura::Window* window,
                                        aura::Window* transient) = 0;
 
  protected:
   virtual ~TransientWindowObserver() {}
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_TRANSIENT_WINDOW_OBSERVER_H_