Add a script to print out stale TestExpectations entries.

This is mostly just to have something checked in so we can print this
list regularly. Eventually, we might want to hook this up to a bot to
nag people who add lines and then don't do anything with them.

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/28843003

git-svn-id: svn://svn.chromium.org/blink/trunk@160108 bbb929c8-8fbe-4397-9dbb-9b2b20218538