[14] RFR 8237369: Shenandoah: failed vmTestbase/nsk/jvmti/AttachOnDemand/attach021/TestDescription.java test

[Zhengyu Gu]

Please review this small patch. keep_alive is only applicable during 
marking phase.


Bug: https://bugs.openjdk.java.net/browse/JDK-8237369
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237369/webrev.00/

Test:
   hotspot_gc_shenandoah

   vmTestbase/nsk/jvmti/AttachOnDemand/attach021/TestDescription.java
   with Shenandoah GC (normal and traversal mode)


Thank,

-Zhengyu