In file js/ipc/WrapperOwner.cpp
Checks if an incoming nsISupports is an nsIDocShellTreeItem (using QueryInterface), and if so returns a static string.
The only uses of this is to generate a tag string that is passed around places with CPOWs, and never really used by anyone except for equality checks relating to CPOWs.
This should likely change to simply checking for the nsIDocShell interface, and if so returning the literal string “ContentDocShell”
As nothing else reads that string (except the comparison operator), it’s unlikely that any tests will break if you change it.