Make modp_b64 and base::base64 compile on Win64

base/base64.cc

 // Copyright (c) 2012 The Chromium 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 "base/base64.h"
 
 #include "third_party/modp_b64/modp_b64.h"
 
 namespace base {
 
 bool Base64Encode(const StringPiece& input, std::string* output) {
   std::string temp;
   temp.resize(modp_b64_encode_len(input.size()));  // makes room for null byte
 
   // null terminates result since result is base64 text!
   int input_size = static_cast<int>(input.size());
 
   // modp_b64_encode_len() returns at least 1, so temp[0] is safe to use.
-  int output_size = modp_b64_encode(&(temp[0]), input.data(), input_size);
-  if (output_size < 0)
+  size_t output_size = modp_b64_encode(&(temp[0]), input.data(), input_size);
+  if (output_size == MODP_B64_ERROR)
     return false;
 
   temp.resize(output_size);  // strips off null byte
   output->swap(temp);
   return true;
 }
 
 bool Base64Decode(const StringPiece& input, std::string* output) {
   std::string temp;
   temp.resize(modp_b64_decode_len(input.size()));
 
   // does not null terminate result since result is binary data!
-  int input_size = static_cast<int>(input.size());
-  int output_size = modp_b64_decode(&(temp[0]), input.data(), input_size);
-  if (output_size < 0)
+  size_t input_size = input.size();
+  size_t output_size = modp_b64_decode(&(temp[0]), input.data(), input_size);
+  if (output_size == MODP_B64_ERROR)
     return false;
 
   temp.resize(output_size);
   output->swap(temp);
   return true;
 }
 
 }  // namespace base