Prefer jinja2 from third_party over eventual system jinja2 libraries.

Source/core/scripts/template_expander.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 13 matching lines @@
 # DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 # THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import os
 import sys
 
 _current_dir = os.path.dirname(os.path.realpath(__file__))
 # jinja2 is in chromium's third_party directory
-sys.path.append(os.path.join(_current_dir, *([os.pardir] * 4)))
+sys.path.insert(0, os.path.join(_current_dir, *([os.pardir] * 4)))
 import jinja2
 
 
 def apply_template(path_to_template, params):
     dirname, basename = os.path.split(path_to_template)
     path_to_templates = os.path.join(_current_dir, "templates")
     jinja_env = jinja2.Environment(loader=jinja2.FileSystemLoader([dirname, path_to_templates]))
     template = jinja_env.get_template(basename)
     return template.render(params)
 
 
 def use_jinja(template_file_name):
     def real_decorator(generator):
         def generator_internal(*args, **kwargs):
             parameters = generator(*args, **kwargs)
             return apply_template(template_file_name, parameters)
         generator_internal.func_name = generator.func_name
         return generator_internal
     return real_decorator