[jfx20u] RFR: 8299977: Update WebKit to 615.1

[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:
 - 8299977: Update WebKit to 615.1

Changes: https://git.openjdk.org/jfx20u/pull/3/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx20u&pr=3&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299977
  Stats: 197124 lines in 5579 files changed: 116283 ins; 42814 del; 38027 mod
  Patch: https://git.openjdk.org/jfx20u/pull/3.diff
  Fetch: git fetch https://git.openjdk.org/jfx20u pull/3/head:pull/3

PR: https://git.openjdk.org/jfx20u/pull/3