Fix "value possibly truncated" warnings on MSVC, net/base edition.

net/base/upload_bytes_element_reader.cc

Index: net/base/upload_bytes_element_reader.cc
diff --git a/net/base/upload_bytes_element_reader.cc b/net/base/upload_bytes_element_reader.cc
index 6927d8ea7c27b5c3ccc1109d7e6525cd1041d10a..44f61f4f8490b9738133812d388a2d2a69abcd38 100644
--- a/net/base/upload_bytes_element_reader.cc
+++ b/net/base/upload_bytes_element_reader.cc
@@ -48,8 +48,8 @@ int UploadBytesElementReader::Read(IOBuffer* buf,
                                    const CompletionCallback& callback) {
   DCHECK_LT(0, buf_length);
 
-  const size_t num_bytes_to_read =
-      std::min(BytesRemaining(), static_cast<uint64>(buf_length));
+  const int num_bytes_to_read = static_cast<int>(
+      std::min(BytesRemaining(), static_cast<uint64>(buf_length)));
 
   // Check if we have anything to copy first, because we are getting
   // the address of an element in |bytes_| and that will throw an