SA: Show Object Histogram asserts with ZGC

The HSDB Show Object Histogram command asserts when using ZGC. It should report that ZGC is unsupported.

How to reproduce:

  1. Run debuggee with -XX:+UnlockExperimentalVMOptions -XX:+UseZGC

  2. Attach HSDB to the debuggee
    $ jhsdb hsdb --pid <PID>

  3. Show Object Histogram
    [Tools] -> [Object Histogram]


Exception on HSDB console:

```
sun.jvm.hotspot.utilities.AssertionFailure: Unexpected CollectedHeap type: sun.jvm.hotspot.gc.z.ZCollectedHeap
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.Assert.that(Assert.java:32)
at jdk.hotspot.agent/sun.jvm.hotspot.oops.ObjectHeap.collectLiveRegions(ObjectHeap.java:448)
at jdk.hotspot.agent/sun.jvm.hotspot.oops.ObjectHeap.iterate(ObjectHeap.java:173)
at jdk.hotspot.agent/sun.jvm.hotspot.HSDB$VisitHeap.run(HSDB.java:1741)
at jdk.hotspot.agent/sun.jvm.hotspot.utilities.WorkerThread$MainLoop.run(WorkerThread.java:70)
at java.base/java.lang.Thread.run(Thread.java:832)
```