RFR: Make UseShenandoahGC experimental

[Roman Kennke]

Yes. Thank you!
Roman

Am 26.09.18 um 12:51 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/experimental-flag/webrev.01/
> 
> I am pretty sure upstream would ask us to turn UseShenandoahGC "experimental", to align with Epsilon
> and Z upstream. This change does it, and make sure tests are still executing fine. When we decide to
> turn the option back to "product", no further changes would be needed.
> 
> Testing: tier3_gc_shenandoah
> 
> Thanks,
> -Aleksey
>