Switch to standard integer types in tools/.

BUG=138542
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Review URL: https://codereview.chromium.org/1549203002

Cr-Commit-Position: refs/heads/master@{#366895}
diff --git a/tools/gn/unique_vector.h b/tools/gn/unique_vector.h
index b5bc65d..e629ce2 100644
--- a/tools/gn/unique_vector.h
+++ b/tools/gn/unique_vector.h
@@ -5,6 +5,8 @@
 #ifndef TOOLS_GN_UNIQUE_VECTOR_H_
 #define TOOLS_GN_UNIQUE_VECTOR_H_
 
+#include <stddef.h>
+
 #include <algorithm>
 
 #include "base/containers/hash_tables.h"