[16] RFR 8248227: Shenandoah: Refactor Shenandoah::heap() to match other GCs

[Zhengyu Gu]

Please review this small patch that refactors Shenandoah::heap() to 
match other GCs.

Bug: https://bugs.openjdk.java.net/browse/JDK-8248227
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8248227/webev.00/index.html


Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu