Use enum class for subtypes of CPDF_Annot.

fpdfsdk/include/cpdfsdk_xfawidget.h

 // Copyright 2016 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_INCLUDE_CPDFSDK_XFAWIDGET_H_
 #define FPDFSDK_INCLUDE_CPDFSDK_XFAWIDGET_H_
 
 #include "core/fxcrt/include/fx_coordinates.h"
 #include "core/fxcrt/include/fx_string.h"
 #include "fpdfsdk/include/cpdfsdk_annot.h"
 
 class CPDFSDK_InterForm;
 class CPDFSDK_PageView;
 class CXFA_FFWidget;
 
 class CPDFSDK_XFAWidget : public CPDFSDK_Annot {
  public:
   CPDFSDK_XFAWidget(CXFA_FFWidget* pAnnot,
                     CPDFSDK_PageView* pPageView,
                     CPDFSDK_InterForm* pInterForm);
   ~CPDFSDK_XFAWidget() override {}
 
   FX_BOOL IsXFAField() override;
   CXFA_FFWidget* GetXFAWidget() const override;
-  CFX_ByteString GetAnnotSubtype() const override;
+  CPDF_Annot::Subtype GetAnnotSubtype() const override;
   CFX_FloatRect GetRect() const override;
 
   CPDFSDK_InterForm* GetInterForm() { return m_pInterForm; }
 
  private:
   CPDFSDK_InterForm* m_pInterForm;
   CXFA_FFWidget* m_hXFAWidget;
 };
 
 #endif  // FPDFSDK_INCLUDE_CPDFSDK_XFAWIDGET_H_