Add support for customizing GN's args text.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 8a433bcdbb1cacf67fd0e96bdbaf5f5bee9dc360..436f82e09ac233304f2809baccd59620aac2791b 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -106,6 +106,10 @@ Variables
 
       The secondary source root must be inside the main source tree.
 
+  arg_file_template [optional]

[brettw, 2016/11/22 16:14:32]

These parameters are alphabetical. Can you put this at the top?

[Dirk Pranke, 2016/11/22 18:41:52]

Done. I thought I checked for that but clearly misread something.

+      Label of a file that contains the text that should be used as the

[brettw, 2016/11/22 16:14:32]

Label -> File name (labels are used for targets not files).

[Dirk Pranke, 2016/11/22 18:41:52]

Changed "Label" to "Path to", instead, and fixed buildconfig to do the same (it
also said label, which is where I got this from).

+      default args.gn file when you run `gn args`.
+
 Example .gn file contents
 
   buildconfig = "//build/config/BUILDCONFIG.gn"
@@ -738,5 +742,22 @@ bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
     build_settings_.set_exec_script_whitelist(std::move(whitelist));
   }
 
+  const Value* arg_file_template_value =
+    dotfile_scope_.GetValue("arg_file_template", true);

[brettw, 2016/11/22 16:14:32]

Indent more.

[Dirk Pranke, 2016/11/22 18:41:52]

Done.

+  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());
+    if (!base::PathExists(path.Resolve(build_settings_.root_path()))) {

[brettw, 2016/11/22 16:14:32]

I don't think we should be doing filesystem operations at this point. This will
almost never be used and you already have some error handling when it is. This
code is always called for every run.

[Dirk Pranke, 2016/11/22 18:41:52]

Okay. I moved the error handling code.

+      err = Err(Location(), arg_file_template_value->string_value() +
+                std::string(" does not exist."));
+      err.PrintToStdout();
+      return false;
+    }
+    build_settings_.set_arg_file_template_path(path);
+  }
+
   return true;
 }