Make modp_b64 and base::base64 compile on Win64

BUG=166496

Review URL: https://chromiumcodereview.appspot.com/11688006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@174700 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/base64.cc b/base/base64.cc
index 1907978..9514b0a 100644
--- a/base/base64.cc
+++ b/base/base64.cc
@@ -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);