Switch breakpoint name from data attribute to content attribute.

It is generally accepted that the content attribute represents the value of the meta element.

```html
<meta name="breakpoint" content="phone" media="(max-width: 480px)">
```

http://www.whatwg.org/specs/web-apps/current-work/multipage/semantics.html#the-meta-element
http://www.w3.org/wiki/HTML/Elements/meta
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta

> name — Metadata name
> content — Value of the element