Fix some sign comparison warnings.

fpdfsdk/src/javascript/Document.cpp

Index: fpdfsdk/src/javascript/Document.cpp
diff --git a/fpdfsdk/src/javascript/Document.cpp b/fpdfsdk/src/javascript/Document.cpp
index 9d231581df1ccd0f62ed2ea0928bc0c960bb9dde..7f078e5a5ecef8c1e1dfb0ef93dfde378a84391d 100644
--- a/fpdfsdk/src/javascript/Document.cpp
+++ b/fpdfsdk/src/javascript/Document.cpp
@@ -1208,7 +1208,7 @@ CFX_WideString Document::CutString(CFX_WideString cbFrom) {
   wchar_t* pResult = (wchar_t*)malloc(iSize.ValueOrDie());
   wchar_t* pFrom = (wchar_t*)cbFrom.GetBuffer(iLength);
 
-  for (int i = 0; i < iLength; i++) {
+  for (size_t i = 0; i < iLength; i++) {
     if (pFrom[i] == L'\\' || pFrom[i] == L'/') {
       pResult[i] = L'\0';
       break;