Prefer third_party version of jinja2

mojo/public/tools/bindings/pylib/mojom/generate/template_expander.py

 # Copyright 2013 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.
 
 # Based on:
 # http://src.chromium.org/viewvc/blink/trunk/Source/build/scripts/template_expander.py
 
 import imp
 import inspect
 import os.path
 import sys
 
 # Disable lint check for finding modules:
 # pylint: disable=F0401
 
 def _GetDirAbove(dirname):
   """Returns the directory "above" this file containing |dirname| (which must
   also be "above" this file)."""
   path = os.path.abspath(__file__)
   while True:
     path, tail = os.path.split(path)
     assert tail
     if tail == dirname:
       return path
 
-try:
-  imp.find_module("jinja2")
-except ImportError:
-  sys.path.append(os.path.join(_GetDirAbove("mojo"), "third_party"))
+sys.path.insert(0, os.path.join(_GetDirAbove("mojo"), "third_party"))
 import jinja2
 
 
 def ApplyTemplate(base_dir, path_to_template, params, filters=None):
   template_directory, template_name = os.path.split(path_to_template)
   path_to_templates = os.path.join(base_dir, template_directory)
   loader = jinja2.FileSystemLoader([path_to_templates])
   jinja_env = jinja2.Environment(loader=loader, keep_trailing_newline=True)
   if filters:
     jinja_env.filters.update(filters)
   template = jinja_env.get_template(template_name)
   return template.render(params)
 
 
 def UseJinja(path_to_template, filters=None):
   # Get the directory of our caller's file.
   base_dir = os.path.dirname(inspect.getfile(sys._getframe(1)))
   def RealDecorator(generator):
     def GeneratorInternal(*args, **kwargs):
       parameters = generator(*args, **kwargs)
       return ApplyTemplate(base_dir, path_to_template, parameters,
                            filters=filters)
     GeneratorInternal.func_name = generator.func_name
     return GeneratorInternal
   return RealDecorator