REGRESSION(CMake): [GTK] WebKitSettings:enable-smooth-scrolling does nothing
https://bugs.webkit.org/show_bug.cgi?id=137781

Reviewed by Martin Robinson.

We used to enable smooth scrolling unconditionally in autotools
(via WebKitFeatures.m4), but since the switch to CMake it's
unconditionally disabled, so changing the setting doesn't have any
effect.

* Source/cmake/OptionsGTK.cmake: Enable smooth scrolling.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174812 268f45cc-cd09-0410-ab3c-d52691b4dbfc