Add support for customizing GN's args text.

tools/gn/setup.cc

 // 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.
 
 #include "tools/gn/setup.h"
 
 #include <stdlib.h>
 #include <algorithm>
 #include <sstream>
 #include <utility>
@@ skipping 40 matching lines @@
   buildfile, but with very limited build setup-specific meaning.
 
   If you specify --root, by default GN will look for the file .gn in that
   directory. If you want to specify a different file, you can additionally pass
   --dotfile:
 
     gn gen out/Debug --root=/home/build --dotfile=/home/my_gn_file.gn
 
 Variables
 
+  arg_file_template [optional]
+      Path to a file containing the text that should be used as the default
+      args.gn content when you run `gn args`.
+
   buildconfig [required]
-      Label of the build config file. This file will be used to set up the
+      Path to the build config file. This file will be used to set up the
       build file execution environment for each toolchain.
 
   check_targets [optional]
       A list of labels and label patterns that should be checked when running
       "gn check" or "gn gen --check". If unspecified, all targets will be
       checked. If it is the empty list, no targets will be checked.
 
       The format of this list is identical to that of "visibility" so see "gn
       help visibility" for examples.
 
@@ skipping 710 matching lines @@
       dotfile_scope_.GetValue("default_args", true);
   if (default_args_value) {
     if (!default_args_value->VerifyTypeIs(Value::SCOPE, &err)) {
       err.PrintToStdout();
       return false;
     }
 
     default_args_ = default_args_value->scope_value();
   }
 
+  const Value* arg_file_template_value =
+      dotfile_scope_.GetValue("arg_file_template", true);
+  if (arg_file_template_value) {
+    if (!arg_file_template_value->VerifyTypeIs(Value::STRING, &err)) {
+      err.PrintToStdout();
+      return false;
+    }
+    SourceFile path(arg_file_template_value->string_value());
+    build_settings_.set_arg_file_template_path(path);
+  }
+
   return true;
 }