Don't use g_return_if_fail when checking read-only
Don't use g_return_if_fail when checking read-only

The previous commit checked the read-only property when initialising the
plugin window and returned if it was. Except I used g_return_if_fail,
which causes asserts. Now we just do a normal check.

clutter-project/clutter-mozembed,clutter-project/clutter-mozembed,clutter-project/clutter-mozembed