I cannot find where it is ever set. Code in ThunkGenerators.cpp looks like it is expected to be used, but that was disabled with a FIXME in <https://bugs.webkit.org/show_bug.cgi?id=127201>
Created attachment 224086 [details] Patch
Comment on attachment 224086 [details] Patch r=me
Committed r164039: <http://trac.webkit.org/changeset/164039>