[jfx11u] RFR: 8302294: Cherry-pick WebKit 615.1 stabilization fixes

[Kevin Rushforth]

Clean backport to `jfx11u`. Tested in connection with all other WebKit backports in `jfx-kcr-11.0.19` branch.

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Commit messages:
 - 8302294: Cherry-pick WebKit 615.1 stabilization fixes

Changes: https://git.openjdk.org/jfx11u/pull/135/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx11u&pr=135&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/jfx11u/pull/135.diff
  Fetch: git fetch https://git.openjdk.org/jfx11u pull/135/head:pull/135

PR: https://git.openjdk.org/jfx11u/pull/135