Add support for customizing GN's args text.

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 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.
 secondary_source = "//build/secondary/"
 
 # These are the targets to check headers for by default. The files in targets
 # matching these patterns (see "gn help label_pattern" for format) will have
 # their includes checked for proper dependencies when you run either
 # "gn check" or "gn gen --check".
 check_targets = [
@@ skipping 227 matching lines @@
       "//third_party/google_input_tools/inputview.gni",
 
       # CLD2 should be removed soon, delete this when we do.
       "//third_party/cld_2/BUILD.gn",
       "//tools/grit/grit_rule.gni",
 
       # Not gypi-to-gn.
       "//google_apis/BUILD.gn",
       "//printing/BUILD.gn",
     ]