The G1CMKeepAliveAndDrainClosure's performance can be improved to not try to call the very expensive drain closure if there has not been any change to the mark bitmap when trying to keep alive the given object, i.e. the mark bitmap has not changed.

Also if the given object is NULL, there is no need to do anything.

Note that with current settings the closure does not do any stealing anyway, so this is always a win.