RFR: 8216377: memory leak for initial nodes of Window

[Florian Kirmaier]

Hi everyone,

While working on an update for JPro <https://www.jpro.one/>, I've found
out, that a unit test for a memory leak doesn't work on JavaFX11.
It's a regression and doesn't happen on JavaFX8. There are more details in
the pull request.

Please review the fix for The Bug:
https://bugs.openjdk.java.net/browse/JDK-8216377
https://github.com/javafxports/openjdk-jfx/pull/342

Florian Kirmaier