We're seeing a hang inside WebKit::CallbackMap in Safari, and we suspect it could be caused by a hash table corruption. Add release assertions to make sure this hash table isn't used in a non-main thread and empty value isn't used as a key.
<rdar://problem/32911286>
Created attachment 313564 [details] Adds assertion
Committed r218676: <http://trac.webkit.org/changeset/218676>