Fix beta warning appearing for some users that it shouldn't.

Check that the user is anonymous before looking at their cookie to decide whether
to show beta warning dialog.
With the existing code, we would see that the preference is false and then check
that they have no cookie. But being logged in should make the cookie existence
be disregarded.

Fixes Ica9e5a92

Bug: 65821
Change-Id: I84e31323930c404222388bb74e4b4ca8d303e05c
Merge "Fix beta warning appearing for some users that it shouldn't." into wmf/1.24wmf7