RFR: 8241840: Memoryleak: Closed focused Stages are not collected with Monocle.

Mon Mar 30 13:47:12 UTC 2020

Closed focused Stages are not collected with Monocle

This commit adds a unit-test and a fix for collecting focused closed stages.

ticket: https://bugs.openjdk.java.net/browse/JDK-8241840

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Commit messages:
 - fixing accidentally commited test
 - cleanup
 - JDK-8241840

Changes: https://git.openjdk.java.net/jfx/pull/153/files
 Webrev: https://webrevs.openjdk.java.net/jfx/153/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8241840
  Stats: 147 lines in 3 files changed: 145 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/153.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/153/head:pull/153

PR: https://git.openjdk.java.net/jfx/pull/153