Introduce GN config files.

.gn

Index: .gn
diff --git a/.gn b/.gn
new file mode 100644
index 0000000000000000000000000000000000000000..6242679953870e26c7917cf90d1d240b3c79a105
--- /dev/null
+++ b/.gn
@@ -0,0 +1,13 @@
+# Copyright 2017 Google Inc. All Rights Reserved.
+
+# 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.
+
+# The location of the build configuration file.
+buildconfig = "//build/config/BUILDCONFIG.gn"
+
+# 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/"