IWYU in testing/range_set.h.

Change-Id: I8d5ae99b22520c3695e026c0ab79b570d1965d52
Reviewed-on: https://pdfium-review.googlesource.com/21810
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/range_set.h b/testing/range_set.h
index 87cbee9..6ed24bd 100644
--- a/testing/range_set.h
+++ b/testing/range_set.h
@@ -5,6 +5,8 @@
 #ifndef TESTING_RANGE_SET_H_
 #define TESTING_RANGE_SET_H_
 
+#include <stddef.h>
+
 #include <set>
 #include <utility>