Add ftplugin for gn filetype

tools/gn/misc/vim/ftplugin/gn.vim

Index: tools/gn/misc/vim/ftplugin/gn.vim
diff --git a/tools/gn/misc/vim/ftplugin/gn.vim b/tools/gn/misc/vim/ftplugin/gn.vim
new file mode 100644
index 0000000000000000000000000000000000000000..ede251dfe99fd14cba0be773fdbfd506b86a3202
--- /dev/null
+++ b/tools/gn/misc/vim/ftplugin/gn.vim
@@ -0,0 +1,12 @@
+" 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.
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+setlocal includeexpr=gn#TranslateToBuildFile(v:fname)
+
+setlocal commentstring=#\ %s