Incorrect question. It's up the implementation detail, up to the particular browser. I don't see any situation when it could be used (perhaps except research). A correct application should not and cannot rely on such information.
[EDIT]
Also, the application should not rely on particular implementation of a browser. If the location of data is known for some particular browser (for WebKit, anyone could learn it using its open source code,
http://www.webkit.org/building/checkout.html[
^]), using it makes no sense anyway.
—SA