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..78e25645d02c774c0b28142a239fe580d8fe8635 100644
--- a/tools/gn/target_generator.h
+++ b/tools/gn/target_generator.h
@@ -54,6 +54,13 @@ class TargetGenerator {
   void FillConfigs();
   void FillOutputs();
 
+  // 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_;
   const FunctionCallNode* function_call_;