But cmake allow makefile generation even it is turn OFF. So later in compilation step i got error: getContentsAsMHTMLData was not defined.
Pull request: https://github.com/WebKit/WebKit/pull/9243
As per comment at https://github.com/WebKit/WebKit/pull/9243#pullrequestreview-1273283595, this is not a bug and should be closed. This is not a flag intended for public use. In *.cmake files, check for fields PUBLIC vs PRIVATE. Touch only the fields marked PUBLIC.