Make toolbar group labels spans instead of divs

The mix of <div> and <a> tags, being floated left inside a floated left
group seems to cause a flow error in some browsers, which ends up
wrapping the last item in the group. The give-away to what was going on
was that this only happened for groups with labels.

While diagnosing it, I notice that the bug did not occur if the labels
were hidden (suggesting is has something to do with the label being
included in the flow of the group, not the build-out), and converting
them to spans fixed the problem.

Bug: 27698
Change-Id: I2a842a86ef77a8934095c04408b7fabbcfbb2476