RFR: 8255527: Shenandoah: Let ShenadoahGCStateResetter disable barriers

[Roman Kennke]

JDK-8255036 introduced a regression: the new ShenandoahGCStateResetter does not actually disable barriers: it only records old state (without changing it) and then restores that state (i.e a no-op). 

Testing: hotspot_gc_shenandoah (also on conc-weakref branch, where the bug did manifest)

-------------

Commit messages:
 - 8255527: Shenandoah: Let ShenadoahGCStateResetter disable barriers

Changes: https://git.openjdk.java.net/jdk/pull/899/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=899&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255527
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/899/head:pull/899

PR: https://git.openjdk.java.net/jdk/pull/899