[Code Coverage] Add requirements.txt for the script.

TBR=inferno@chromium.org,liaoyuke@chromium.org

Bug: 817798
Change-Id: Id38a434ce51619e66962a789eb6d4c808a11bda0
Reviewed-on: https://chromium-review.googlesource.com/1183524
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584733}

tools/code_coverage/requirements.txt

+Jinja2==2.10