Fix access after free on shutdown

After we keep around the browser-context after the profile is deleted
it was keeping pointers to deleted objects and would sometimes use them
on shutdown.

Change-Id: Ib67d0ee0b27cb1a1b64d9b8b4c348ed418b9bbc3
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>