RFR (XS): 8055091: CollectedHeap::_reserved usage should be cleaned up

[Marcus Larsson]

Hi,

Can I have reviews for this small change to make the _reserved field of 
CollectedHeap private, unifying its initialization in a protected method.

CR:
http://cr.openjdk.java.net/~mgerdin/mlarsson/webrev-8055091/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8055091

Testing:
jprt

Thanks,
Marcus