This is a known bug upstream: https://bugzilla.gnome.org/show_bug.cgi?id=693403 it is fixed after gnome-shell 3.7.91. However, it is breaking current F19 composes and thus is an F19 Alpha blocker, so we kinda need to track it. If 3.7.92 does not land soon or gnome-screensaver is not added to comps temporarily, all F19 composes will fail to start Shell. Proposing as an Alpha blocker: this prevents the default desktop from running. New-style criterion "Release-blocking live images must boot to the expected boot menu, and then to a desktop or to a login prompt where it is clear how to log in to a desktop" - you can't get to a desktop on the live image.
GNOME Shell 3.7.92 is in F19 now, and I've verified the fix works in my latest live images (I dropped the addition of gnome-screensaver, and they still work).