RFR: 8144701: Change three G1 remembererd set closures to be OopClosures

[Stefan Johansson]

Hi,

Please review this change for:
https://bugs.openjdk.java.net/browse/JDK-8144701

Webrev:
http://cr.openjdk.java.net/~sjohanss/8138888/8144701/hotspot.00

Summary:
While working on JDK-8138888 and its sub-tasks I found three closures 
that inherited ExtendedOopClosure even though they were only used as 
normal OopClosures. I've changed them to inherit OopClosure and removed 
them from the list of closures in g1_specialized_oop_closures.hpp.

Testing:
* RBT (currently running)

Thanks,
Stefan