RFR(T) 8227101: Shenandoah: Use ShenandoahSharedFlag for claimed flag in ShenandoahSerialRoot

[Zhengyu Gu]

Please review this trivial patch, that uses ShenandoahSharedFlag for 
claimed flag in ShenandoahSerialRoot, to be consistent with 
ShenandoahWeakSerialRoot.


Bug: https://bugs.openjdk.java.net/browse/JDK-8227101
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8227101/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug  and release)

Thanks,

-Zhengyu