Add support for customizing GN's args text.

build/sample_arg_file.gn

Index: build/sample_arg_file.gn
diff --git a/build/sample_arg_file.gn b/build/sample_arg_file.gn
new file mode 100644
index 0000000000000000000000000000000000000000..5e5df24967d97b1d26f52b5f27195c1f1af7fb81
--- /dev/null
+++ b/build/sample_arg_file.gn
@@ -0,0 +1,6 @@
+# Build arguments go here. Here are some of the most commonly set arguments,
+# and their default values (as of when `gn args` was run):

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

I think you you delete this second line. The first line is descriptive enough,
and the second line is misleading (is_component_build doesn't default to true if
you flip is_debug).

If we want to add something else, I'd add:
  Run "gn args --list <build_dir>" to see available args.

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

Done.

+#   is_debug = true
+#   is_component_build = true
+#   symbol_level = 2
+#   use_goma = false

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

This is kind of a dumb file name, but I didn't have any better ideas, so I'm
open to suggestions. I'm also open to suggestions for the contents of this file.