Make modp_b64 and base::base64 compile on Win64

base/base64.cc

Index: base/base64.cc
===================================================================
--- base/base64.cc	(revision 174668)
+++ base/base64.cc	(working copy)
@@ -16,8 +16,8 @@
   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
@@ -30,9 +30,9 @@
   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);