Use enum class for subtypes of CPDF_Annot.

fpdfsdk/cpdfsdk_annot.cpp

 // 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
 
 #include "fpdfsdk/include/cpdfsdk_annot.h"
 
 #include <algorithm>
 
@@ skipping 40 matching lines @@
 }
 
 int CPDFSDK_Annot::GetLayoutOrder() const {
   return 5;
 }
 
 CPDF_Annot* CPDFSDK_Annot::GetPDFAnnot() const {
   return nullptr;
 }
 
-CFX_ByteString CPDFSDK_Annot::GetAnnotSubtype() const {
-  return "";
+CPDF_Annot::Subtype CPDFSDK_Annot::GetAnnotSubtype() const {
+  return CPDF_Annot::Subtype::UNKNOWN;
 }
 
 bool CPDFSDK_Annot::IsSignatureWidget() const {
   return false;
 }
 
 void CPDFSDK_Annot::SetRect(const CFX_FloatRect& rect) {}
 
 CFX_FloatRect CPDFSDK_Annot::GetRect() const {
   return CFX_FloatRect();
@@ skipping 15 matching lines @@
 #ifdef PDF_ENABLE_XFA
   return GetPDFXFAPage();
 #else   // PDF_ENABLE_XFA
   return GetPDFPage();
 #endif  // PDF_ENABLE_XFA
 }
 
 CPDF_Page* CPDFSDK_Annot::GetPDFPage() {
   return m_pPageView ? m_pPageView->GetPDFPage() : nullptr;
 }