Add support for customizing GN's args text.

.gn

Index: .gn
diff --git a/.gn b/.gn
index 45305c8b99a232772363b352f81896f82fbdd1e8..a3136d7832110ced3ec987abb507d7092ecb0bfc 100644
--- a/.gn
+++ b/.gn
@@ -7,6 +7,10 @@ import("//build/dotfile_settings.gni")
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
+# This points to a file containing the text to pre-fill the editor
+# with when you run `gn args`.
+arg_file_template = "//build/sample_arg_file.gn"

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

We don't want to actually land the change to this file until after GN is rolled,
but I'm including it for illustrative purposes for now.

+
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.