RFR: JDK-8269092: Add OldObjectSampleEvent.allocationSize field

[Ludovic Henry]

The data is already available in ObjectSample.allocated and getting this data
in another way is complicated (via Instrumentation for example). Having the
size of the objects also allows to augment the memory leak analysis based on
the object size impact on top of based on the object count impact.

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Commit messages:
 - JDK-8269092: Add OldObjectSampleEvent.allocationSize field

Changes: https://git.openjdk.java.net/jdk/pull/4549/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4549&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269092
  Stats: 94 lines in 3 files changed: 94 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4549.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4549/head:pull/4549

PR: https://git.openjdk.java.net/jdk/pull/4549