RFR: 8252381: Cherry pick GTK WebKit 2.28.4 changes

[Bhawesh Choudhary]

On Thu, 27 Aug 2020 21:27:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Update to GTK WebKit 2.28.4
>> https://webkitgtk.org/2020/07/28/webkitgtk2.28.4-released.html
>
> Looks good. Also, I tested on all three platforms.

Verified with all three platform. Looks good.

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