RFR: 8263840: PeriodicTask should declare its destructor virtual

[David Holmes]

Please review this trivial fix.

Testing tiers 1-3 as sanity check

Thanks,
David

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Commit messages:
 - 8263840: PeriodicTask should declare its destructor virtual

Changes: https://git.openjdk.java.net/jdk/pull/4886/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4886&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263840
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4886.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4886/head:pull/4886

PR: https://git.openjdk.java.net/jdk/pull/4886