Add support for customizing GN's args text.

tools/gn/build_settings.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_BUILD_SETTINGS_H_
 #define TOOLS_GN_BUILD_SETTINGS_H_
 
 #include <map>
 #include <memory>
 #include <set>
@@ skipping 34 matching lines @@
   }
   void SetSecondarySourcePath(const SourceDir& d);
 
   // Path of the python executable to run scripts with.
   base::FilePath python_path() const { return python_path_; }
   void set_python_path(const base::FilePath& p) { python_path_ = p; }
 
   const SourceFile& build_config_file() const { return build_config_file_; }
   void set_build_config_file(const SourceFile& f) { build_config_file_ = f; }
 
+  // Path to a file containing the default text to use when running `gn args`.
+  const SourceFile& arg_file_template_path() const {
+    return arg_file_template_path_;
+  }
+  void set_arg_file_template_path(const SourceFile& f) {
+    arg_file_template_path_ = f;
+  }
+
   // The build directory is the root of all output files. The default toolchain
   // files go into here, and non-default toolchains will have separate
   // toolchain-specific root directories inside this.
   const SourceDir& build_dir() const { return build_dir_; }
   void SetBuildDir(const SourceDir& dir);
 
   // The build args are normally specified on the command-line.
   Args& build_args() { return build_args_; }
   const Args& build_args() const { return build_args_; }
 
@@ skipping 30 matching lines @@
     exec_script_whitelist_ = std::move(list);
   }
 
  private:
   base::FilePath root_path_;
   std::string root_path_utf8_;
   base::FilePath secondary_source_path_;
   base::FilePath python_path_;
 
   SourceFile build_config_file_;
+  SourceFile arg_file_template_path_;
   SourceDir build_dir_;
   Args build_args_;
 
   ItemDefinedCallback item_defined_callback_;
   PrintCallback print_callback_;
 
   std::unique_ptr<std::set<SourceFile>> exec_script_whitelist_;
 
   BuildSettings& operator=(const BuildSettings& other);  // Disallow.
 };
 
 #endif  // TOOLS_GN_BUILD_SETTINGS_H_