Add media_perftests to trybot runs.

There have been instances where CQ has let changes through that
break media_perftests (e.g., http://crbug.com/376578).

Review URL: https://codereview.chromium.org/322503003

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@278982 4ff67af0-8c30-449e-8e8b-ad334ec8d88c