unbundle: add shim_headers.gni and unbundled GN file for zlib

build/linux/unbundle/replace_gn_files.py

 #!/usr/bin/env python
 # Copyright 2016 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.
 
 """
 Replaces GN files in tree with files from here that
 make the build use system libraries.
 """
 
 from __future__ import print_function
 
 import argparse
 import os
 import shutil
 import sys
 
 
 REPLACEMENTS = {
   'libxml': 'third_party/libxml/BUILD.gn',
+  'zlib': 'third_party/zlib/BUILD.gn',
 }
 
 
 def DoMain(argv):
   my_dirname = os.path.dirname(__file__)
   source_tree_root = os.path.abspath(
     os.path.join(my_dirname, '..', '..', '..'))
 
   parser = argparse.ArgumentParser()
   parser.add_argument('--system-libraries', nargs='*', default=[])
@@ skipping 25 matching lines @@
   if unhandled_libraries:
     print('Unrecognized system libraries requested: %s' % ', '.join(
         sorted(unhandled_libraries)), file=sys.stderr)
     return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(DoMain(sys.argv[1:]))