[jfx20u] RFR: 8301712: [linux] Crash on exit from WebKit 615.1

[Kevin Rushforth]

Clean backport to `jfx20u`. Tested in connection with all other WebKit backports in `jfx-20.0.1` branch.

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Commit messages:
 - 8301712: [linux] Crash on exit from WebKit 615.1

Changes: https://git.openjdk.org/jfx20u/pull/4/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx20u&pr=4&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301712
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx20u/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/jfx20u pull/4/head:pull/4

PR: https://git.openjdk.org/jfx20u/pull/4