[RFR] 8209049: Cherry pick GTK WebKit 2.20.4 changes

[Arunprasad Rajkumar]

Hi Kevin/Murali,

Please review the fix the following bug,

https://bugs.openjdk.java.net/browse/JDK-8209049

I didn’t generate webrev because patch contains changes from GTK WebKit 2.20.4 
branch[1].

[1] https://trac.webkit.org/log/webkit/releases/WebKitGTK/webkit-2.20

Thanks,
Arun