Reduce signed/unsigned comparison warnings

testing/fx_string_testhelpers.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.
 
 #include "testing/fx_string_testhelpers.h"
 
 #include <iomanip>
 #include <ios>
 
 namespace {
 
 template <typename T>
 std::ostream& output_string(std::ostream& out, const T& str) {
   out << std::hex << std::setfill('0') << '"';
-  for (size_t i = 0; i < str.GetLength(); ++i) {
+  // This function is used for FX strings whose length is defined as int.
+  for (int i = 0; i < str.GetLength(); ++i) {
     unsigned int c = str.GetAt(i);
     if (c >= 0x20 && c < 0x7F) {
       out << static_cast<char>(c);
     } else if (sizeof(typename T::value_type) == 1) {
       out << "\\x" << std::setw(2) << c << std::setw(0);
     } else if (c < 0x10000) {
       out << "\\u" << std::setw(4) << c << std::setw(0);
     } else {
       out << "<invalid>";
     }
@@ skipping 12 matching lines @@
   return output_string(out, str);
 }
 
 std::ostream& operator<<(std::ostream& out, const CFX_WideStringC& str) {
   return output_string(out, str);
 }
 
 std::ostream& operator<<(std::ostream& out, const CFX_WideString& str) {
   return output_string(out, str);
 }