RFR: (sh/8): [backport] 8226757: Shenandoah: Make traversal and passive modes explicit

[Zhengyu Gu]

Ok.

-Zhengyu

On 11/11/19 9:56 AM, Roman Kennke wrote:
> This backports the machinery that introduces ShenandoahGCMode, in
> preparation for the Traversal GC backport. Contrary to the corresponding
> 11+ changes, this does not include the traversal mode. However, I will
> push this change together with the big traversal GC backport.
> 
> Webrev:
> http://cr.openjdk.java.net/~rkennke/JDK-8226757-jdk8/webrev.00/
> 
> Testing: hotspot_gc_shenandoah
> 
> Can I please get a review?
> 
> Roman
>