bookmarks: Forward declare some classes in header files when possible.

chrome/browser/bookmarks/recently_used_folders_combo_model.h

 // Copyright (c) 2012 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 CHROME_BROWSER_BOOKMARKS_RECENTLY_USED_FOLDERS_COMBO_MODEL_H_
 #define CHROME_BROWSER_BOOKMARKS_RECENTLY_USED_FOLDERS_COMBO_MODEL_H_
 
 #include <vector>
 
-#include "base/string16.h"
-#include "chrome/browser/bookmarks/bookmark_model.h"
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
 #include "ui/base/models/combobox_model.h"
 
+class BookmarkModel;
+class BookmarkNode;
+
 // Model for the combobox showing the list of folders to choose from. The
 // list always contains the bookmark bar, other node and parent. The list
 // also contains an extra item that shows the text 'Choose another folder...'.
 class RecentlyUsedFoldersComboModel : public ui::ComboboxModel {
  public:
   RecentlyUsedFoldersComboModel(BookmarkModel* model, const BookmarkNode* node);
   virtual ~RecentlyUsedFoldersComboModel();
 
   // Overridden from ui::ComboboxModel:
   virtual int GetItemCount() const OVERRIDE;
   virtual string16 GetItemAt(int index) OVERRIDE;
 
   // Returns the node at the specified |index|.
   const BookmarkNode* GetNodeAt(int index);
 
   // Returns the index of the original parent folder.
   int node_parent_index() const { return node_parent_index_; }
 
  private:
   // Removes |node| from |nodes_|. Does nothing if |node| is not in |nodes_|.
   void RemoveNode(const BookmarkNode* node);
 
   std::vector<const BookmarkNode*> nodes_;
   int node_parent_index_;
 
   DISALLOW_COPY_AND_ASSIGN(RecentlyUsedFoldersComboModel);
 };
 
 #endif  // CHROME_BROWSER_BOOKMARKS_RECENTLY_USED_FOLDERS_COMBO_MODEL_H_