Computed bindings are recomputed (say, on a model update), when the
arguments to the computation function change. Since no arguments were
being passed to changeURL(), it was only being computed once.
In order to maintain a clean API for external callers, move changeURL
from a function to a computed property. This way the caller doesn't
have to pass in the change number but it still updates when the change
number does.
Bug: Issue 3689
Change-Id: I92a659b3c917f77d4e212c98af7c763174756074
05a7a5727c