RFR: JDK-8299977 : Update WebKit to 615.1

[Hima Bindu Meda]

Update JavaFX WebKit to GTK WebKit 2.38 (615.1).

Verified the updated version build, sanity tests and stability.
This does not cause any issues except one unit test failure.
Also, there are some artifacts observed while playing youtube

The above issues are recorded and ignored using below bugs:
[JDK-8300954](https://bugs.openjdk.org/browse/JDK-8300954)
[JDK-8301022](https://bugs.openjdk.org/browse/JDK-8301022)

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Commit messages:
 - Update Webkit to v615.1

Changes: https://git.openjdk.org/jfx/pull/1011/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1011&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/jfx/pull/1011.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1011/head:pull/1011

PR: https://git.openjdk.org/jfx/pull/1011