RFR: Add ShenandoahRootProcessor API to report threads while scanning roots

[Aleksey Shipilev]

On 01/23/2018 09:24 PM, Roman Kennke wrote:
> As discussed in the Traversal GC thread, this breaks out the ShenandoahRootProcessor API to report
> threads while scanning roots. It is not implemented here, and only asserts that the ThreadClosure*
> is NULL. All call-sites are updated to pass NULL.
> 
> The idea is to make backporting easier/less conflict-prone.
> 
> http://cr.openjdk.java.net/~rkennke/root-proc-threads/webrev.00/
> 
> Test: hotspot_gc_shenandoah
> 
> Ok?

OK!

-Aleksey