RFR: 8138888: Remove ExtendedOopClosure::apply_to_weak_ref_discovered_field

[Stefan Johansson]

Hi,

Please review this change for:
https://bugs.openjdk.java.net/browse/JDK-8138888

Webrev:
http://cr.openjdk.java.net/~sjohanss/8138888/hotspot.00/

Summary:
This change builds upon 8138737 which is currently out for review. With 
that change we now have a more generic mechanism to add special handling 
for closures when visiting InstanceRefKlasses. With this mechanism we 
can remove ExtendedOopClosure::apply_to_weak_ref_discovered_field and 
replace it with a new ReferenceIterationMode which provides the same 
functionality.

The RFE in JBS also proposes to remove the functionality of 
apply_to_weak_ref_discovered_field, but to do this some more analysis is 
needed. Doing this changes is a step in the direction of making it 
easier to remove the need for this functionality in future changes.

Testing:
* JPRT
* RBT hs-tier 2+3

Thanks,
Stefan