Introduce GN config files.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..703f20b7de7885c62908d5d17036425539db02b9
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,16 @@
+# Copyright 2017 The Chromium Authors. All Rights Reserved.
+
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This is the root build file for GN. GN will start processing by loading this
+# file, and recursively load all dependencies until all dependencies are either
+# resolved or known not to exist (which will cause the build to fail). So if
+# you add a new build file, there must be some path of dependencies from this
+# file to your new one or GN won't know about it.
+# In GN, a "group" is a dummy target that just lists other targets.
+
+group("all") {
+  deps = [
+  ]
+}