Reland of Add helpful error messages to check_gn_headers.py

build/check_gn_headers_unittest.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 logging
 import json
 import os
 import unittest
 import check_gn_headers
@@ skipping 26 matching lines @@
    "targets": {
       "//:All": {
       },
       "//:base": {
          "public": [ "//base/p.h" ],
          "sources": [ "//base/a.cc", "//base/a.h", "//base/b.hh" ],
          "visibility": [ "*" ]
       },
       "//:star_public": {
          "public": "*",
-         "sources": [ "//base/c.h" ],
+         "sources": [ "//base/c.h", "//tmp/gen/a.h" ],
          "visibility": [ "*" ]
       }
     }
 }
 ''')
 
 
 whitelist = r'''
    white-front.c
 a/b/c/white-end.c # comment
@@ skipping 25 matching lines @@
         'dir\\path\\b.h',
         'c.hh',
         'dir3\\path\\b.h',
         'c3.hh',
     ])
     self.assertEquals(headers, expected)
 
     os.sep = old_sep
 
   def testGn(self):
-    headers = check_gn_headers.ParseGNProjectJSON(gn_input)
+    headers = check_gn_headers.ParseGNProjectJSON(gn_input,
+                                                  'out/Release', 'tmp')
     expected = set([
         'base/a.h',
         'base/b.hh',
         'base/c.h',
         'base/p.h',
+        'out/Release/gen/a.h',
     ])
     self.assertEquals(headers, expected)
 
   def testWhitelist(self):
     output = check_gn_headers.ParseWhiteList(whitelist)
     expected = set([
         'white-front.c',
         'a/b/c/white-end.c',
         'dir/white-both.c',
         'a/b/c',
     ])
     self.assertEquals(output, expected)
 
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)
   unittest.main(verbosity=2)