RFR: JDK-8304705 Parent class contains 6 constants that should be static

[John Hendrikx]

Made 6 constants static in `Parent`.  Discovered these with the Java Object Layout tool.

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Commit messages:
 - Make constants static

Changes: https://git.openjdk.org/jfx/pull/1064/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1064&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304705
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1064.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1064/head:pull/1064

PR: https://git.openjdk.org/jfx/pull/1064