Make cocoa and nicosia implementations inherit from it and move all the common code there
Pull request: https://github.com/WebKit/WebKit/pull/5625
Committed 255867@main (03b3e7f3ada3): <https://commits.webkit.org/255867@main> Reviewed commits have been landed. Closing PR #5625 and removing active labels.
<rdar://problem/101456114>