[EFL] Add a ENABLE_CSS_SCROLL_SNAP macro to CMake build system

[EFL] Add a ENABLE_CSS_SCROLL_SNAP macro to CMake build system
https://bugs.webkit.org/show_bug.cgi?id=139085

Reviewed by Andreas Kling.

.:

* Source/cmake/OptionsEfl.cmake: Add ENABLE_CSS_SCROLL_SNAP.
* Source/cmake/WebKitFeatures.cmake: ditto.
* Source/cmakeconfig.h.cmake: ditto.

Source/WebCore:

* PlatformEfl.cmake:
    Include page/scrolling/AxisScrollSnapOffsets.cpp to EFL build files to fix build break.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@176620 268f45cc-cd09-0410-ab3c-d52691b4dbfc