[jfx11u] RFR: 8302684: Cherry-pick WebKit 615.1 stabilization fixes (2)

[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:
 - 8302684: Cherry-pick WebKit 615.1 stabilization fixes (2)

Changes: https://git.openjdk.org/jfx11u/pull/136/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx11u&pr=136&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302684
  Stats: 34 lines in 14 files changed: 18 ins; 1 del; 15 mod
  Patch: https://git.openjdk.org/jfx11u/pull/136.diff
  Fetch: git fetch https://git.openjdk.org/jfx11u pull/136/head:pull/136

PR: https://git.openjdk.org/jfx11u/pull/136