RFR(T) 8219976: GarbageCollectionNotificationInfo always says "No GC" when running Shenandoah

[zgu at redhat.com]

Please review this trivial fix, that sets heap's GC cause during GC
session, so the value can be picked up, instead of default one. 

Bug: https://bugs.openjdk.java.net/browse/JDK-8219976
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8219976/webrev.00/

Test:
  tier1, 2, 3_gc_shenandoah on Linux 64 (fastdebug and release)
  

Thanks,

-Zhengyu