RFR (XXS): 8133042: Remove some direct accesses of G1Allocator::reuse_retained_old_region() to G1CollectedHeap members

[Thomas Schatzl]

Hi all,

  could I have reviews of this tiny change that changes some direct
private member accesses of G1Allocator to G1CollectedHeap to some
already available public method calls?

While this is more a cosmetic change, I thought it would be good to
decrease the amount of "friendliness" between those two classes. I could
not find a good other CR to add them, so here we are....

CR:
https://bugs.openjdk.java.net/browse/JDK-8133042
Webrev:
http://cr.openjdk.java.net/~tschatzl/8133042/webrev/
Testing:
local compilation

Thanks,
  Thomas