RFR: (S): 8153117: Back out JDK-8142935 until JDK-8152723 fixed

[Tom Benson]

Hi,
Please take a look at this change to back out the recent G1 change for 
JDK-8142935 (Adding old gen regions does not consider available free 
space) until the side effect reported in JDK-8152723 is fixed. Under 
stress, old gen regions might be shut out of mixed GCs, eventually 
triggering a full GC.

JBS: https://bugs.openjdk.java.net/browse/JDK-8153117
webrev: http://cr.openjdk.java.net/~tbenson/8153117/webrev/
testing: JPRT builds
Thanks,
Tom