GN: Add optional default_args variable to .gn.

tools/gn/setup.h

Index: tools/gn/setup.h
diff --git a/tools/gn/setup.h b/tools/gn/setup.h
index 45335bf5b0e66e5fa9ca0167383e0b507ac5ec5a..d9a77d063a3765f605e23431d78b73d72cb0794b 100644
--- a/tools/gn/setup.h
+++ b/tools/gn/setup.h
@@ -150,6 +150,10 @@ class Setup {
   std::vector<Token> dotfile_tokens_;
   std::unique_ptr<ParseNode> dotfile_root_;
 
+  // Default overrides, specified in the dotfile.
+  // Owned by the Value (if it exists) in the dotfile_scope_.
+  const Scope* default_args_;
+
   // Set to true when we should populate the build arguments from the command
   // line or build argument file. See setter above.
   bool fill_arguments_;