RFR: 8263402: MemoryLeak: Node hardreferences it's previous Parent after csslayout and getting removed from the scene [v4]

[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.

Florian Kirmaier has updated the pull request incrementally with one additional commit since the last revision:

  8263402
  Minor cleanup based on codereview

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/424/files
  - new: https://git.openjdk.java.net/jfx/pull/424/files/52fa05f6..e8063e30

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=424&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=424&range=02-03

  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 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