[jfx11u] RFR: 8088420: JavaFX WebView memory leak via EventListener

[Ambarish Rapte]

Clean backport to jfx11u.

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Commit messages:
 - 8088420: JavaFX WebView memory leak via EventListener

Changes: https://git.openjdk.org/jfx11u/pull/105/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx11u&pr=105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8088420
  Stats: 1509 lines in 11 files changed: 1503 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jfx11u/pull/105.diff
  Fetch: git fetch https://git.openjdk.org/jfx11u pull/105/head:pull/105

PR: https://git.openjdk.org/jfx11u/pull/105