RFR: 8207843: HSDB cannot show Object Histogram when ZGC is working

[Yasumasa Suenaga]

Hi all,

Please review this webrev.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8207843
  webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8207843/webrev.00/

I encountered AssertionFailure when I attached HSDB to the process which is 
working with ZGC as below:

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)

ObjectHeap#collectLiveRegions() branches by instance type of CollectedHeap. 
However it does not support ZCollectedHeap.
So I add ZCollectedHeap to it and add some methods to iterate ZPageTable.


Thanks,

Yasumasa