RFR: 8275848: Deprecate mistakenly exposed field from class javafx.scene.shape.Box

[Ajit Ghaisas]

This PR deprecates mistakenly exposed field from class javafx.scene.shape.Box.

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Commit messages:
 - Deprecate DEFAULT_SIZE constant field

Changes: https://git.openjdk.java.net/jfx/pull/655/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=655&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275848
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/655.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/655/head:pull/655

PR: https://git.openjdk.java.net/jfx/pull/655