Add new model directory to app.yaml.

Missed this in r178597.

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/406973004

git-svn-id: svn://svn.chromium.org/blink/trunk@178598 bbb929c8-8fbe-4397-9dbb-9b2b20218538