It's slightly embarrassing that we haven't noticed this before now, but character outside the BMP are not rendered at all.
Created attachment 185086 [details] Patch
Comment on attachment 185086 [details] Patch LGTM, thanks!
Comment on attachment 185086 [details] Patch Clearing flags on attachment: 185086 Committed r141122: <http://trac.webkit.org/changeset/141122>
All reviewed patches have been landed. Closing bug.