RFR: 8236912: Preventing NPE when clicking WebView with forward/back mouse buttons

[Robert Lichtenberger]

As documented in JDK-8236912, WebView did not check whether the idMap really contained a mapping for the given button, making it prone to errors, when things are extended (as has happened here).

The fix consists of two test cases that show the problem in unfixed WebViews and a fix which works analogously to the check whether the given event type is mapped.

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Commits:
 - 542ff60e: Fix null pointer exception when clicking into WebViews with forward/back mouse button.
 - 2109d5a0: Merge remote-tracking branch 'upstream/master'
 - acfa63be: Merge remote-tracking branch 'upstream/master'
 - 7c5cf198: 8232524: Test cleanup: terminate background thread upon failure.
 - 7e80839f: 8232524: SynchronizedObservableMap cannot be be protected for copying/iterating
 - 8ecf3545: JDK-8232524 fixed.

Changes: https://git.openjdk.java.net/jfx/pull/85/files
 Webrev: https://webrevs.openjdk.java.net/jfx/85/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236912
  Stats: 18 lines in 2 files changed: 15 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/85.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/85/head:pull/85

PR: https://git.openjdk.java.net/jfx/pull/85