binding: Updates Jinja2 from 2.7.1 to 2.8.

New feature in 2.8:
- block assignment
  {% set var_name %} content {% endset %}

Manually edited files:
  README.chromium
  get_jinja2.sh

BUG=

Review-Url: https://codereview.chromium.org/2316103002
Cr-Commit-Position: refs/heads/master@{#417191}
24 files changed