RFR: 8259647: Add support for JFR event ObjectCountAfterGC to Shenandoah

[Zhengyu Gu]

Please review this patch that adds JFR ObjectCountAfterGC event support.

AFAICT, the event is off by default. If it is enabled, it distorts Shenandoah pause characteristics, since it performs heap walk during final mark pause. 

When event is disabled:
`[191.033s][info][gc,stats] Pause Init Mark (G)                 454 us`
`[191.033s][info][gc,stats] Pause Init Mark (N)                  13 us`

When event is enabled:
`[396.631s][info][gc,stats] Pause Final Mark (G)              43199 us`
`[396.631s][info][gc,stats] Pause Final Mark (N)              42982 us`

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8259647-object_count_jfr

Changes: https://git.openjdk.java.net/jdk/pull/2386/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2386&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259647
  Stats: 12 lines in 3 files changed: 6 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2386.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2386/head:pull/2386

PR: https://git.openjdk.java.net/jdk/pull/2386