gn: Diag on distinct toolchains with the same name.

tools/gn/ninja_build_writer.h

Index: tools/gn/ninja_build_writer.h
diff --git a/tools/gn/ninja_build_writer.h b/tools/gn/ninja_build_writer.h
index 530ac7189035cea75d46c0a36fcdc983dfde6c13..51535aabf76ca16f9d5b6d1c5869a2b9638a4f80 100644
--- a/tools/gn/ninja_build_writer.h
+++ b/tools/gn/ninja_build_writer.h
@@ -49,9 +49,8 @@ class NinjaBuildWriter {
  private:
   void WriteNinjaRules();
   void WriteAllPools();
-  void WriteSubninjas();
-  bool WritePhonyAndAllRules(
-      Err* err);
+  bool WriteSubninjas(Err* err);
+  bool WritePhonyAndAllRules(Err* err);
 
   void WritePhonyRule(const Target* target, const std::string& phony_name);