Consistently use meta_grab_op_is_resizing() for _NET_WM_SYNC_REQUEST

In different places we checked the grab op differently when determing
whether we are using _NET_WM_SYNC_REQUEST. This was somewhat covered
up previously by the fact that we only had a sync alarm when using
_NET_WM_SYNC_REQUEST, but that is no longer the case, so consistently
use meta_grab_op_is_resizing() everywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=685463