It looks like support for extra_flag_values was added before check-webkit-style was forked (in order to support the "git-commit" option). When the "git-commit" option was subsequently moved into ProcessorOptions, the extra_flag_values attribute became no longer necessary.
Created attachment 50199 [details]
Comment on attachment 50199 [details]
Manually committed: http://trac.webkit.org/changeset/55715