Toasts are unclickable (http://stackoverflow.com/a/20698525) so this button
never actually did anything. If we want to add this functionality, we'd have to
use a new toasting API (dialog was suggested) and potentially create a new
Activity (so it can be shown over another underlying Activity).
We could implement this but it'd be better to spend time creating system
notifications to show tab sending progress (bug 1145434).
