Support script response files in GN.

tools/gn/substitution_type.h

Index: tools/gn/substitution_type.h
diff --git a/tools/gn/substitution_type.h b/tools/gn/substitution_type.h
index 42040c9c0be0da5b3428f3113cb3da57a37eba5d..8940a1e76a53b6c07b762c279110f10dd21b39d2 100644
--- a/tools/gn/substitution_type.h
+++ b/tools/gn/substitution_type.h
@@ -58,6 +58,9 @@ enum SubstitutionType {
   SUBSTITUTION_OUTPUT_EXTENSION,  // {{output_extension}}
   SUBSTITUTION_SOLIBS,  // {{solibs}}
 
+  // Used only for the args of actions.
+  SUBSTITUTION_RSP_FILE_NAME,  // {{response_file_name}}
+
   SUBSTITUTION_NUM_TYPES  // Must be last.
 };