RFR: 8144486: Change G1UpdateRSOrPushRefOopClosure to inherit OopClosure

[Stefan Johansson]

Hi,

Please review this change for:
https://bugs.openjdk.java.net/browse/JDK-8144486

Webrev:
http://cr.openjdk.java.net/~sjohanss/8138888/8144486/hotspot.00/

Background:
When iterating objects with oop_iterate() there is a special case for 
InstanceRefKlass, if the closure overrides 
apply_to_weak_ref_discovered_field() to return true, the closure will be 
applied to the discovered field, even if the normal checks prevents it. 
This special treatment is needed in certain situations, but it is not 
obvious if it should be done when iterating the object. Currently only 
G1 closures override apply_to_weak_ref_discovered_field() and many of 
them seem to do it without any reason. My initial plan [1] was to remove 
all usage of this special casing but after discussions I realized it 
would be easier to see the effects if doing this one closure at a time.

Summary:
This change is for G1UpdateRSOrPushRefOopClosure which is never used 
with oop_iterate and therefore does not need to override 
apply_to_weak_ref_discovered_field(). There is also no need to inherit 
ExtendedOopClosure, so it's changed to inherit OopClosure instead.

Testing:
* JPRT
* RBT

Thanks,
Stefan

[1] https://bugs.openjdk.java.net/browse/JDK-8138888