Stop converting widestring -> c_str -> widestring in several places.

fpdfsdk/pdfwindow/PWL_ComboBox.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef FPDFSDK_PDFWINDOW_PWL_COMBOBOX_H_
 #define FPDFSDK_PDFWINDOW_PWL_COMBOBOX_H_
 
 #include "fpdfsdk/pdfwindow/PWL_Edit.h"
@@ skipping 51 matching lines @@
                 intptr_t lParam = 0) override;
   void CreateChildWnd(const PWL_CREATEPARAM& cp) override;
   void RePosChildWnd() override;
   CFX_FloatRect GetFocusRect() const override;
   void SetFocus() override;
   void KillFocus() override;
 
   void SetFillerNotify(IPWL_Filler_Notify* pNotify);
 
   CFX_WideString GetText() const;
-  void SetText(const FX_WCHAR* text);
-
-  void AddString(const FX_WCHAR* str);
+  void SetText(const CFX_WideString& text);
+  void AddString(const CFX_WideString& str);
   int32_t GetSelect() const;
   void SetSelect(int32_t nItemIndex);
 
   void SetEditSel(int32_t nStartChar, int32_t nEndChar);
   void GetEditSel(int32_t& nStartChar, int32_t& nEndChar) const;
   void Clear();
   void SelectAll();
   FX_BOOL IsPopup() const;
 
   void SetSelectText();
@@ skipping 12 matching lines @@
   FX_BOOL m_bPopup;
   CFX_FloatRect m_rcOldWindow;
   int32_t m_nPopupWhere;
   int32_t m_nSelectItem;
   IPWL_Filler_Notify* m_pFillerNotify;
 
   void* m_pFormFiller;
 };
 
 #endif  // FPDFSDK_PDFWINDOW_PWL_COMBOBOX_H_