In GNOME 3, if you try to add a Google account (under the "Online Accounts" section of the settings), the control center gets killed. Using "paxctl -m /usr/bin/gnome-control-center" fixes that for me. (Of course there are many other security issues here, including the fact that this feature hasn't worked for users with Google's two-factor authentication for some time. So I won't be using this feature anytime soon regardless, but I figured I'd report it.)
I suspect the same issue here as in bug #493636. In any case, please attach the output of "emerge --info webkit-gtk gnome-online-accounts gnome-control-center".
Created attachment 367108 [details] output of emerge --info webkit-gtk gnome-online-accounts gnome-control-center
*** This bug has been marked as a duplicate of bug 484300 ***