RFR: 8219332: ZGC: Improve ZRootsIteratorClosure abstraction

[Stefan Karlsson]

Looks good.

StefanK

On 2019-02-19 14:40, Per Liden wrote:
> Remove the need to call the super class in all implementors of 
> ZRootsIteratorClosure::do_thread() .
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8219332
> Webrev: http://cr.openjdk.java.net/~pliden/8219332/webrev.0
> 
> /Per