RFR: 8047821: G1 Does not use the save_marks functionality as intended

[Mikael Gerdin]

Hi!

As part of a larger effort to detach G1's HeapRegion from ContiguousSpace[1]
and as a general cleanup we should rename the save_marks and set_saved_marks
methods on HeapRegion. They are not used with oops_since_saved_marks_iterate
and cause more confusion than anything. 

This change is part of a set of 4 changes: 8047818, 8047819, 8047820, 8047821 
which are needed to refactor the HeapRegion class and its superclasses 
in order to simplify the G1 class unloading change which is coming.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8047821
Webrev:
http://cr.openjdk.java.net/~mgerdin/8047821/webrev/

[1] https://bugs.openjdk.java.net/browse/JDK-8047818

Thanks
/Mikael