Change how presence of VisualEditor is determined

Meaning of isAvailable changed, so now just check for the VE tab,
which suits the purpose here (seeing if the VE tour is usable).

Bug: 60111
Change-Id: Ibaa701d89924aec4804d453f62a0b685a67d95f1
(cherry picked from commit 97e4abcd40b0ebe434316878100371a58f01e04e)