RFR: JDK-4305789: BasicToolBarUI: Request conversion of private instance variable

[aLiEnHeAd13]

Fixed Bug JDK-4305789 to support docking to any instantiated JFrame https://bugs.java.com/bugdatabase/view_bug.do?bug_id=4305789

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Commit messages:
 - Update BasicToolBarUI.java

Changes: https://git.openjdk.org/jdk/pull/10309/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4305789
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10309.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10309/head:pull/10309

PR: https://git.openjdk.org/jdk/pull/10309