RFR: 8081474: SwingWorker calls 'done' before the 'doInBackground' is finished [v4]

[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.

Prasanta Sadhukhan has updated the pull request incrementally with one additional commit since the last revision:

  Fix and test updated

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11940/files
  - new: https://git.openjdk.org/jdk/pull/11940/files/4f59bfa4..c34d55b9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11940&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11940&range=02-03

  Stats: 26 lines in 2 files changed: 3 ins; 15 del; 8 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