According to Darin Adler, the idiom for making a temporary 16-bit copy of a string is to use StringView::upconvertedCharacters(). (https://bugs.webkit.org/show_bug.cgi?id=147604#c9)
Created attachment 265013 [details] Patch
Comment on attachment 265013 [details] Patch r=me
Comment on attachment 265013 [details] Patch Clearing flags on attachment: 265013 Committed r192146: <http://trac.webkit.org/changeset/192146>
All reviewed patches have been landed. Closing bug.