RFR(S): 7143490: G1: Remove HeapRegion::_top_at_conc_mark_count

[John Cuthbertson]

Hi Everyone,

Here's my contribution to the cleanup week.

Can I have a couple of volunteers review the changes for this CR? The 
webrev can be found at: http://cr.openjdk.java.net/~johnc/7143490/webrev.0/

Summary:
As a result of Tony's recent marking changes (associated with the 
marking of survivors) the field HeapRegion::_top_at_conc_mark_count is 
no longer required. The value of _top_at_conc_mark_count is now the same 
as NTAMS. Additionally I simplified the verification code that sets the 
bits in the live card bitmap and refactored the closures that finalize 
and verify the liveness counting data to avoid code duplication.

Testing: I inserted a 500ms sleep in the marking cycle between the 
remark and cleanup pauses to ensure that at least one evacuation pause 
was seen during this phase, and ran the GC test suite with several 
marking thresholds (2, 5, and 10%) and verification enabled.

Thanks,

JohnC