_after_ we've popped the JSContext from the stack, and to handle multiple termination functions being posted from a single script evaluation. Fixes leaks when closing windows or tabs with still-loading documents. Bug 295983, r+sr=jst, a=asa git-svn-id: svn://10.0.0.236/trunk@174364 18797224-902f-48f8-a5cc-f745e15eee43