RFR: 8293444: Creating ScrollPane with same content component causes memory leak

[Andy Goryachev]

Using Weak*Listeners eliminates the memory leak.

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Commit messages:
 - 8293444: test
 - 8293444: using weak listeners

Changes: https://git.openjdk.org/jfx/pull/900/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=900&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293444
  Stats: 88 lines in 2 files changed: 75 ins; 4 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/900.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/900/head:pull/900

PR: https://git.openjdk.org/jfx/pull/900