RFR: 8289388: Fix warnings: method is overriding a synchronized method without being synchronized

[Andy Goryachev]

Fixes warnings generated by the latest Eclipse: 
method is overriding a synchronized method without being synchronized

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

Commit messages:
 - 8289388: fixed method is overriding a synchronized method warning

Changes: https://git.openjdk.org/jfx/pull/819/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=819&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289388
  Stats: 7 lines in 3 files changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/819.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/819/head:pull/819

PR: https://git.openjdk.org/jfx/pull/819