RFR: JDK-8176520: Improve the accuracy of the instance size in hprof heap dumps

[Alex Menkov]

The fix updates heap dumpers to report correct instance size value for HPROF_GC_CLASS_DUMP records (currently it's reported as size of all instance fields)

Testing: tier1, tier2, tier5-svc

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Commit messages:
 - jcheck
 - fix

Changes: https://git.openjdk.org/jdk/pull/17855/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17855&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8176520
  Stats: 175 lines in 6 files changed: 141 ins; 21 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/17855.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17855/head:pull/17855

PR: https://git.openjdk.org/jdk/pull/17855