GN: Add optional default_args variable to .gn.

tools/gn/setup.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_SETUP_H_
 #define TOOLS_GN_SETUP_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 132 matching lines @@
   // dot file.
   Settings dotfile_settings_;
   Scope dotfile_scope_;
 
   // State for invoking the dotfile.
   base::FilePath dotfile_name_;
   std::unique_ptr<InputFile> dotfile_input_file_;
   std::vector<Token> dotfile_tokens_;
   std::unique_ptr<ParseNode> dotfile_root_;
 
+  // Default overrides, specified in the dotfile.

[brettw, 2017/01/19 18:47:08]

Can you add a comment here that this pointer is owned by the value (if it
exists) in the dotfile_scope_?

[timn, 2017/01/21 02:52:45]

Done.

+  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_;
 
   // State for invoking the command line args. We specifically want to keep
   // this around for the entire run so that Values can blame to the command
   // line when we issue errors about them.
   std::unique_ptr<InputFile> args_input_file_;
   std::vector<Token> args_tokens_;
   std::unique_ptr<ParseNode> args_root_;
 
   DISALLOW_COPY_AND_ASSIGN(Setup);
 };
 
 #endif  // TOOLS_GN_SETUP_H_