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

fpdfsdk/pdfwindow/PWL_ComboBox.h

Index: fpdfsdk/pdfwindow/PWL_ComboBox.h
diff --git a/fpdfsdk/pdfwindow/PWL_ComboBox.h b/fpdfsdk/pdfwindow/PWL_ComboBox.h
index 02f9d1380a192c507dd574a2ef202a6f7e676a0d..df9eb5546e8523941d64c3cfe118a1354b75a939 100644
--- a/fpdfsdk/pdfwindow/PWL_ComboBox.h
+++ b/fpdfsdk/pdfwindow/PWL_ComboBox.h
@@ -69,9 +69,8 @@ class CPWL_ComboBox : public CPWL_Wnd {
   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);