RFR (S): 6490394: G1: Allow heap shrinking / memory unmapping after reclaiming regions during Remark

[Thomas Schatzl]

Hi all,

  allow me to provide a fix for the oldest g1-specific open issue on
the bug tracker (from 2006 :)): resize the heap (eg. uncommit)
according to existing sizing policies at remark time.

It does exactly what the subject of the CR says.

The feature is enabled by default - I did not see any reason to disable
it as the functionality seems to actually be what would be expected of
the concurrent cycle (as it mirrors the functionality of a full gc).

Needs the change for JDK-8071913 currently out for review applied to
work (i.e. not crash after uncommit).

The current work for "JDK-8204089: Timely Reducing Unused Committed
Memory" also depends on this change.

CR:
https://bugs.openjdk.java.net/browse/JDK-6490394
Webrev:
http://cr.openjdk.java.net/~tschatzl/6490394/webrev/
Testing:
hs-tier1-5, jdk-tier1-3, perf testing ongoing but not expecting a
difference

Thanks,
  Thomas