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

[Roman Kennke]

Looks good to me.

Thanks!
Roman

> 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
>