RFR: 8144505: Change G1ParCopyHelper to inherit OopClosure

[Stefan Johansson]

Hi,

Please review this change for:
https://bugs.openjdk.java.net/browse/JDK-8144505

Webrev:
http://cr.openjdk.java.net/~sjohanss/8138888/8144505/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:
G1ParCopyClosure is only used as a normal OopClosure. It inherits from 
G1ParCopyHelper which in turn inherits G1ParClosureSuper. 
G1ParClosureSuper overrides apply_to_weak_ref_discovered_field() to 
return true but since G1ParCopyClosure is never used with oop_iterate() 
it doesn't need to inherit this functionality. G1ParCopyHelper can 
instead be changed to inherit directly from OopClosure, and this way we 
have one closure less depending on apply_to_weak_ref_discovered_field().

Testing:
* JPRT
* RBT

Thanks,
Stefan

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