RFR: 8276337: Use override specifier in HeapDumper

[Leo Korinth]

Use override specifier in HeapDumper. It is safer.

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Commit messages:
 - 8276337: Use override specifier in HeapDumper

Changes: https://git.openjdk.java.net/jdk/pull/6274/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6274&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276337
  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6274.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6274/head:pull/6274

PR: https://git.openjdk.java.net/jdk/pull/6274