Add OptionSetTraits<> and use with WebCore::DragDestinationAction. This change was split out from Bug 212115 Attachment #400121 [details] (Patch v4).
Waiting to post patch until fix for Bug 212115 lands.
<rdar://problem/63669015>
Created attachment 400341 [details] Patch v1
Committed r262216: <https://trac.webkit.org/changeset/262216> All reviewed patches have been landed. Closing bug and clearing flags on attachment 400341 [details].