[CMake] Options should be marked as advanced by default

[CMake] Options should be marked as advanced by default
https://bugs.webkit.org/show_bug.cgi?id=143572

Reviewed by Gyuyoung Kim.

Options defined with WEBKIT_OPTION_DEFINE are now advanced so that they are hidden from
users by default, unless WEBKIT_OPTION_DEFAULT_PORT_VALUE is used. Add new macros
WEBKIT_OPTION_DEFINE_PUBLIC to define an option that's not hidden by default, to be used
for adding port-specific options, and WEBKIT_OPTION_PRIVATE_PORT_VALUE to override an
option without making it public.

* Source/cmake/WebKitFeatures.cmake:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182658 268f45cc-cd09-0410-ab3c-d52691b4dbfc