[15] RFR 8246162: Shenandoah: full GC does not mark code roots when class unloading is off

[Zhengyu Gu]

When class unloading is off, Shenandoah should mark through code roots. 
In normal cycle, this is done during concurrent marking phase. However, 
full GC does not execute concurrent marking phase, we should piggyback 
code root marking in final mark.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246162
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8246162/webrev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu