RFR: 8081474: SwingWorker calls 'done' before the 'doInBackground' is finished

[Prasanta Sadhukhan]

SwingWorker done() method [spec ](https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/SwingWorker.java#L452) says "Executed on the Event Dispatch Thread after the doInBackground method is finished"
but there's no mechanism in place to honor that claim.
The [spec](https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/SwingWorker.java#L289)
also says the state should be DONE after doInBackground() returns which is also not done.

Modified the code to honour the specification.

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Commit messages:
 - Fix
 - Fix
 - 8081474: SwingWorker calls 'done' before the 'doInBackground' is finished

Changes: https://git.openjdk.org/jdk/pull/11940/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11940&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8081474
  Stats: 96 lines in 2 files changed: 93 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11940.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11940/head:pull/11940

PR: https://git.openjdk.org/jdk/pull/11940