RFR: 8377099: Additional WebKit 623.1 fixes from WebKitGTK 2.50.4

[Hima Bindu Meda]

This PR updates webkit to v2.50.4. Verified build and sanity on mac, windows and linux platforms. No issue seen.

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Commit messages:
 - additional fixes from webkit-2.50.4

Changes: https://git.openjdk.org/jfx/pull/2063/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2063&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377099
  Stats: 692 lines in 75 files changed: 467 ins; 72 del; 153 mod
  Patch: https://git.openjdk.org/jfx/pull/2063.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2063/head:pull/2063

PR: https://git.openjdk.org/jfx/pull/2063