RFR: 8353916: Unexpected event type for DOM mutation events with WebKit 620.1

[Jay Bhaskar]

Issue: In WebKit 619.1, MutationEvent was properly handled and overridden, so casting to it worked as expected. In newer WebKit versions (like the one embedded in current JavaFX), MutationEvent is deprecated, and the actual event dispatched is just a generic DOM Event.
Solution: restore the old style mutation event support, which will cause the returned events to again be instances of MutationEvent.

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Commit messages:
 - Merge branch 'openjdk:master' into mutevent
 - mute evnt fix

Changes: https://git.openjdk.org/jfx/pull/1765/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1765&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353916
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1765.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1765/head:pull/1765

PR: https://git.openjdk.org/jfx/pull/1765