Fix GN action target writing with no sources list.

tools/gn/substitution_writer.h

Index: tools/gn/substitution_writer.h
diff --git a/tools/gn/substitution_writer.h b/tools/gn/substitution_writer.h
index 2bc9ed2aa86ebe3e41af2a6a9dd9e3f783ef7d9b..11b8eb60ec238a235a06a54b9a00b3ec464bae87 100644
--- a/tools/gn/substitution_writer.h
+++ b/tools/gn/substitution_writer.h
@@ -33,6 +33,19 @@ class SubstitutionWriter {
   SubstitutionWriter();
   ~SubstitutionWriter();
 
+  // Converts the given SubstitutionList to OutputFiles assuming there are
+  // no substitutions (it will assert if there are). This is used for cases
+  // like actions where the outputs are explicit, but the list is stored as
+  // a SubstitutionList.
+  static void GetListAsSourceFiles(
+      const Settings* settings,
+      const SubstitutionList& list,
+      std::vector<SourceFile>* output);
+  static void GetListAsOutputFiles(
+      const Settings* settings,
+      const SubstitutionList& list,
+      std::vector<OutputFile>* output);
+
   // Applies the substitution pattern to a source file, returning the result
   // as either a SourceFile or OutputFile.
   static SourceFile ApplyPatternToSource(