vulcanize_gn.py: Stop writing twice to the declared GN output file.

chrome/browser/resources/vulcanize_gn_test.py

 #!/usr/bin/env python
 # 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.
 
 import os
 import shutil
 import tempfile
 import unittest
 import vulcanize_gn
@@ skipping 53 matching lines @@
 
     self._run_vulcanize(depfile='depfile.d',
                         html_in_file='ui.html',
                         html_out_file='fast.html',
                         js_out_file='fast.js')
 
     fast_html = self._read_out_file('fast.html')
     self.assertFalse('element.html' in fast_html)
     self.assertFalse('element.js' in fast_html)
     self.assertTrue('got here!' in fast_html)
+    self.assertTrue('<script src="fast.js"></script>' in fast_html)
 
     fast_js = self._read_out_file('fast.js')
     self.assertTrue('yay' in fast_js)
 
     depfile_d = self._read_out_file('depfile.d')
     self.assertTrue('element.html' in depfile_d)
     self.assertTrue('element.js' in depfile_d)
 
 
 if __name__ == '__main__':
   unittest.main()