JDK-8255269 incorrectly replaced average copy time

The patch for JDK-8255269 incorrectly replaced the average copy time computation with the one from JDK 11. This line from the patch

double average_copy_time = average_time_ms(G1GCPhaseTimes::ObjCopy);

should be

double average_copy_time = average_time_ms(G1GCPhaseTimes::ObjCopy) + average_time_ms(G1GCPhaseTimes::OptObjCopy);