[jfx18] RFR: 8281711: Cherry-pick WebKit 613.1 stabilization fixes

[Johan Vos]

Stabilization fixes from WebKitGTK 2.34.5

Reviewed-by: jvos, kcr

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Commit messages:
 - 8281711: Cherry-pick WebKit 613.1 stabilization fixes

Changes: https://git.openjdk.java.net/jfx/pull/773/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=773&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281711
  Stats: 1218 lines in 127 files changed: 767 ins; 169 del; 282 mod
  Patch: https://git.openjdk.java.net/jfx/pull/773.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/773/head:pull/773

PR: https://git.openjdk.java.net/jfx/pull/773