paper-tabs: setting center=true squashes tabs / breaks pagination

[pauln]

If tabs are rendered with center=true on a small screen, they get squashed up to fit within the visible space, preventing pagination from kicking in:

This can be seen by adding center: true to the demo/tabs.js controller in the dummy app and loading it in a narrow browser (i.e. on a mobile device or using mobile view in dev tools).

I'm guessing that this hasn't been noticed yet because if center is toggled after page load (such as by ticking the center box in the docs) or you open the page in a larger window and then resize to mobile size, the tabs appear full size and paginated, as expected:

[panthony]

I think this issue is fixed on master