For some reason the confirmEx prompt behaves differently (as a modal
window?) on Ubuntu if it's the only window open, preventing the
setTimeout() callback within support.js::waitForWindow() from being run.
(Passing -c to the test runner fixed it.) This removes a win.close() in
the middle of the file, which was probably a mistake anyway, so that the
pref window stays open until the end of the file.
Improves UX of sync authentication.
The account is now linked and unlinked and an API key related to
the client is generated transparently in the background.
The API key is deleted on unlinking.
No sync options are allowed before linking an account.
