RFR: 8249839: Cherry pick GTK WebKit 2.28.3 changes

[Arun Joseph]

Update to GTK WebKit 2.28.3
https://webkitgtk.org/2020/07/09/webkitgtk2.28.3-released.html

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Commit messages:
 - Cherry pick GTK WebKit 2.28.3 changes

Changes: https://git.openjdk.java.net/jfx/pull/271/files
 Webrev: https://webrevs.openjdk.java.net/jfx/271/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249839
  Stats: 533 lines in 36 files changed: 381 ins; 66 del; 86 mod
  Patch: https://git.openjdk.java.net/jfx/pull/271.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/271/head:pull/271

PR: https://git.openjdk.java.net/jfx/pull/271