Add support for customizing GN's args text.

tools/gn/command_args.cc

Index: tools/gn/command_args.cc
diff --git a/tools/gn/command_args.cc b/tools/gn/command_args.cc
index 1b81135be3d9ff23e632380568d956c8ec4661b5..9ff28cedb1c3994939a3e92dc218a671604237f5 100644
--- a/tools/gn/command_args.cc
+++ b/tools/gn/command_args.cc
@@ -243,12 +243,27 @@ int EditArgsFile(const std::string& build_dir) {
         setup.build_settings().GetFullPath(setup.GetBuildArgFile())
         .NormalizePathSeparators();
     if (!base::PathExists(arg_file)) {
-      std::string argfile_default_contents =
-          "# Build arguments go here. Examples:\n"
-          "#   is_component_build = true\n"
-          "#   is_debug = false\n"
-          "# See \"gn args <out_dir> --list\" for available build "
-          "arguments.\n";
+      BuildSettings build_settings = setup.build_settings();
+      base::FilePath arg_file_template_path =
+        build_settings.GetFullPath(build_settings.arg_file_template_path())

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

Indent more.

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

Done.

+        .NormalizePathSeparators();
+
+      std::string argfile_default_contents;
+      if (!build_settings.arg_file_template_path().is_null()) {
+        base::FilePath arg_file_path_template_path =
+          build_settings.GetFullPath(build_settings.arg_file_template_path());

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

Indent mode (maybe this will have to wrap, sadly).

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

Reworked to be shorter.

+        if (!base::ReadFileToString(arg_file_template_path,
+                                    &argfile_default_contents)) {
+          return 1;

[Dirk Pranke, 2016/11/22 04:27:39]

Should I print an err to stdout here if this read fails?

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

No, but I would prefer if it fell through to the default contents.

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

Done. I'm not sure if I'm wild about that, but it's probably also not usually
going to happen.

+        }
+      } else {
+        argfile_default_contents =
+            "# Build arguments go here. Examples:\n"
+            "#   is_component_build = true\n"
+            "#   is_debug = false\n"

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

Can you remove the examples in this case?

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

Done.

+            "# See \"gn args <out_dir> --list\" for available build "
+            "arguments.\n";
+      }
 #if defined(OS_WIN)
       // Use Windows lineendings for this file since it will often open in
       // Notepad which can't handle Unix ones.