Add optional <DEPS file> argument to roll-dep.

BUG=chromium:425117
R=agable@chromium.org,kjellander@chromium.org

Review URL: https://codereview.chromium.org/666713004

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292563 4ff67af0-8c30-449e-8e8b-ad334ec8d88c