make size_t->int explicit

Change-Id: I9a5dc268fed845cd354e575e9155221caa1c4a14
diff --git a/tools/obu_parser.cc b/tools/obu_parser.cc
index fff51d5..1c24c4c 100644
--- a/tools/obu_parser.cc
+++ b/tools/obu_parser.cc
@@ -183,7 +183,7 @@
     aom_uleb_decode(data + consumed + obu_header_size, remaining, &obu_size,
                     &length_field_size);
     current_obu_length = static_cast<int>(obu_size);
-    consumed += obu_header_size + length_field_size + current_obu_length;
+    consumed += obu_header_size + (int)length_field_size + current_obu_length;
     printf("      length:      %d\n",
            static_cast<int>(obu_header_size + length_field_size +
                             current_obu_length));