RFR: 8263402: MemoryLeak: Node hardreferences it's previous Parent after csslayout and getting removed from the scene

[Florian Kirmaier]

Fixing a memory leak. 
A node hard references its old parent after CSS layout and getting removed. 
This shouldn't be the case, this is very counterintuitive.

The fix uses a WeakReference in CSSStyleHelper for firstStyleableAncestor.
This should be fine because the CSS should only depend on it if it's still the real parent. 
In that case, it doesn't get collected.

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Commit messages:
 - 8263402

Changes: https://git.openjdk.java.net/jfx/pull/424/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=424&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263402
  Stats: 116 lines in 2 files changed: 107 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jfx/pull/424.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/424/head:pull/424

PR: https://git.openjdk.java.net/jfx/pull/424