Prefer jinja2 from third_party over eventual system jinja2 libraries.

We recently added keep_trailing_newline which requires jinja2.7. The one in
third_party is 2.7.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23498033

git-svn-id: svn://svn.chromium.org/blink/trunk@157768 bbb929c8-8fbe-4397-9dbb-9b2b20218538