Switch to standard integer types in extensions/.

BUG=138542
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Review URL: https://codereview.chromium.org/1543053002

Cr-Commit-Position: refs/heads/master@{#366716}
diff --git a/extensions/utility/unpacker.cc b/extensions/utility/unpacker.cc
index 4763268..be0a4b5 100644
--- a/extensions/utility/unpacker.cc
+++ b/extensions/utility/unpacker.cc
@@ -4,6 +4,8 @@
 
 #include "extensions/utility/unpacker.h"
 
+#include <stddef.h>
+
 #include <set>
 #include <utility>