ui: Forward declare DefaultSingletonTraits in header files.

ui/views/focus/view_storage.h

 // 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_VIEWS_FOCUS_VIEW_STORAGE_H_
 #define UI_VIEWS_FOCUS_VIEW_STORAGE_H_
 
-#include "base/memory/singleton.h"
-#include "ui/views/view.h"
+#include <map>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "ui/views/views_export.h"
+
+template <typename T> struct DefaultSingletonTraits;
 
 // This class is a simple storage place for storing/retrieving views.  It is
 // used for example in the FocusManager to store/restore focused views when the
 // main window becomes active/inactive.
 // It automatically removes a view from the storage if the view is removed from
 // the tree hierarchy.
 //
 // To use it, you first need to create a view storage id that can then be used
 // to store/retrieve views.
 
 namespace views {
+class View;
 
 class VIEWS_EXPORT ViewStorage {
  public:
   // Returns the global ViewStorage instance.
   // It is guaranted to be non NULL.
   static ViewStorage* GetInstance();
 
   // Returns a unique storage id that can be used to store/retrieve views.
   int CreateStorageID();
 
@@ skipping 29 matching lines @@
 
   // Association View to id, used to speed up view notification removal.
   std::map<View*, std::vector<int>*> view_to_ids_;
 
   DISALLOW_COPY_AND_ASSIGN(ViewStorage);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_FOCUS_VIEW_STORAGE_H_