RFR: 8252381: Cherry pick GTK WebKit 2.28.4 changes

[Arun Joseph]

Update to GTK WebKit 2.28.4
https://webkitgtk.org/2020/07/28/webkitgtk2.28.4-released.html

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Commit messages:
 - 8252381: Cherry pick GTK WebKit 2.28.4 changes

Changes: https://git.openjdk.java.net/jfx/pull/289/files
 Webrev: https://webrevs.openjdk.java.net/jfx/289/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252381
  Stats: 174 lines in 21 files changed: 123 ins; 16 del; 35 mod
  Patch: https://git.openjdk.java.net/jfx/pull/289.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/289/head:pull/289

PR: https://git.openjdk.java.net/jfx/pull/289