Add ftplugin for gn filetype

tools/gn/misc/vim/autoload/gn.vim

Index: tools/gn/misc/vim/autoload/gn.vim
diff --git a/tools/gn/misc/vim/autoload/gn.vim b/tools/gn/misc/vim/autoload/gn.vim
new file mode 100644
index 0000000000000000000000000000000000000000..c1df012149b944e1bcceefde79f5a74467c5efe0
--- /dev/null
+++ b/tools/gn/misc/vim/autoload/gn.vim
@@ -0,0 +1,17 @@
+" 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.
+
+function! gn#TranslateToBuildFile(name) abort
+  " Strip '//' prefix
+  let l:new_path = substitute(a:name, '\v^//', '', '')
+
+  " Strip the build target name (necessary if 'isfname' contains ':')
+  let l:new_path = substitute(l:new_path, '\v:.*$', '', '')
+
+  " Append 'BUILD.gn', only if this is a directory and not a file
+  if isdirectory(l:new_path)
+    let l:new_path = substitute(l:new_path, '\v/?$', '/BUILD.gn', '')
+  endif
+  return l:new_path
+endfunction