Fix the build on Mac.

Review URL: http://codereview.chromium.org/8354037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9726 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/serialize.cc

@@ -1158,7 +1158,8 @@ void Serializer::VisitPointers(Object** start, Object** end) {
 
   for (Object** current = start; current < end; current++) {
     if (start == isolate->heap()->roots_array_start()) {
-      root_index_wave_front_ = Max(root_index_wave_front_, current - start);
+      root_index_wave_front_ =
+          Max(root_index_wave_front_, static_cast<intptr_t>(current - start));
     }
     if (reinterpret_cast<Address>(current) ==
         isolate->heap()->store_buffer()->TopAddress()) {