RFR: 8219332: ZGC: Improve ZRootsIteratorClosure abstraction

[Per Liden]

Thanks Stefan!

/Per

On 2/19/19 3:00 PM, Stefan Karlsson wrote:
> 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