RFR: 8238526: Cherry pick GTK WebKit 2.26.3 changes

[Johan Vos]

On Thu, 6 Feb 2020 19:35:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Marked as reviewed by kcr (Lead).
> 
> As part of the review, I did a sanity build / test on all three platforms. All looks good.

build and basic checks ok on mac/linux/win64. Didn't test on win32.

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PR: https://git.openjdk.java.net/jfx/pull/107