RFR 8225014: Separate ShenandoahRootScanner method for object_iterate

[Zhengyu Gu]

Please review this patch that allows SH::object_iterate() invokes 
ShenandoahRootScanner method without assertion. A regression test is 
also added.


Bug: https://bugs.openjdk.java.net/browse/JDK-8225014
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225014/webrev.00/

Test:
   hotspot_gc_shenandoah with new regression test (fastdebug and release)

Thanks,

-Zhengyu