Add a FIXME explaining PageGroup's lack of utility

R=abarth@chromium.org
BUG=299834

Review URL: https://codereview.chromium.org/115333005

git-svn-id: svn://svn.chromium.org/blink/trunk@163924 bbb929c8-8fbe-4397-9dbb-9b2b20218538