[jfx20u] RFR: 8302294: Cherry-pick WebKit 615.1 stabilization fixes

[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:
 - 8302294: Cherry-pick WebKit 615.1 stabilization fixes

Changes: https://git.openjdk.org/jfx20u/pull/5/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx20u&pr=5&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302294
  Stats: 1797 lines in 198 files changed: 834 ins; 508 del; 455 mod
  Patch: https://git.openjdk.org/jfx20u/pull/5.diff
  Fetch: git fetch https://git.openjdk.org/jfx20u pull/5/head:pull/5

PR: https://git.openjdk.org/jfx20u/pull/5