Add ftplugin for gn filetype

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

+" 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:.*$', '', '')
+
+  " This section requires the open source vim-maktaba plugin. If it's not
+  " installed, there's not much we can do here

[scottmg, 2017/01/24 04:22:58]

This seems a bit defeatist. Surely we could append "BUILD.gn" with a relatively
simple conditional without requiring this large dependency in the case of .gn
files?

[Nate Fischer, 2017/01/24 18:12:19]

Ok, I think I found a robust way to do it without the maktaba dep.

+  if exists('*maktaba#path#Basename')
+    " Append 'BUILD.gn', only if this doesn't already end in a filename (this
+    " checks if it has an extension)
+    if maktaba#path#Basename(l:new_path) !~# '\V.'
+      let l:new_path = maktaba#path#Join([l:new_path, 'BUILD.gn'])
+    endif
+  else
+    echoerr 'Please install vim-maktaba'
+  endif
+  return l:new_path
+endfunction