Fix some sign comparison warnings.

fpdfsdk/src/formfiller/FFL_ListBox.cpp

 // 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
 
 #include "../../include/formfiller/FormFiller.h"
 #include "../../include/formfiller/FFL_FormFiller.h"
 #include "../../include/formfiller/FFL_ListBox.h"
 //#include "../../include/formfiller/FFL_Module.h"
@@ skipping 83 matching lines @@
                              FX_UINT nFlags) {
   return CFFL_FormFiller::OnChar(pAnnot, nChar, nFlags);
 }
 
 FX_BOOL CFFL_ListBox::IsDataChanged(CPDFSDK_PageView* pPageView) {
   CPWL_ListBox* pListBox = (CPWL_ListBox*)GetPDFWindow(pPageView, FALSE);
   if (!pListBox)
     return FALSE;
 
   if (m_pWidget->GetFieldFlags() & FIELDFLAG_MULTISELECT) {
-    int nSelCount = 0;
+    size_t nSelCount = 0;
     for (int32_t i = 0, sz = pListBox->GetCount(); i < sz; ++i) {
       if (pListBox->IsItemSelected(i)) {
         if (m_OriginSelections.count(i) == 0)
           return TRUE;
 
-        nSelCount++;
+        ++nSelCount;
       }
     }
 
     return nSelCount != m_OriginSelections.size();
   }
   return pListBox->GetCurSel() != m_pWidget->GetSelectedIndex(0);
 }
 
 void CFFL_ListBox::SaveData(CPDFSDK_PageView* pPageView) {
   ASSERT(m_pWidget != NULL);
@@ skipping 118 matching lines @@
       CPDFSDK_PageView* pPageView = GetCurPageView();
       ASSERT(pPageView != NULL);
 
       if (CommitData(pPageView, nFlag)) {
         DestroyPDFWindow(pPageView);
         m_bValid = FALSE;
       }
     }
   }
 }