RFR: 8269908: Move MemoryService::track_memory_usage call into G1MonitoringScope

[Thomas Schatzl]

Hi all,

  can I have reviews for this tiny change that moves the `MemoryService::track_memory_usage()` call into the destructor of `G1MonitoringScope` so that all jstat/mxbeans related reporting is handled by that single place?

Testing: tier1-3

Thanks,
  Thomas

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/4687/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4687&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269908
  Stats: 4 lines in 2 files changed: 3 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4687.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4687/head:pull/4687

PR: https://git.openjdk.java.net/jdk/pull/4687