Refactor GN source expansions.

tools/gn/target_generator.h

Index: tools/gn/target_generator.h
diff --git a/tools/gn/target_generator.h b/tools/gn/target_generator.h
index baee0f1114ca10a1858f968678b69b11b280f2fe..7428a4dd5eda10a471ea5fa322a04063d4dad105 100644
--- a/tools/gn/target_generator.h
+++ b/tools/gn/target_generator.h
@@ -52,7 +52,14 @@ class TargetGenerator {
   void FillPublic();
   void FillInputs();
   void FillConfigs();
-  void FillOutputs();
+  void FillOutputs(bool allow_substitutions);
+
+  // Rrturns true if the given pattern will expand to a file in the output
+  // directory. If not, returns false and sets the error, blaming the given
+  // Value.
+  bool EnsureSubstitutionIsInOutputDir(
+      const SubstitutionPattern& pattern,
+      const Value& original_value);
 
   Target* target_;
   Scope* scope_;