[cssom-view] Need to define how fractional lengths are converted to integers.

There are various APIs in the OM which return integer sizes. Some of them:

• Window.{inner,outer}{Width,Height}.
• Window.screen{X,Y,Top,Left}.
• Screen.avail{Width,Height}.
• Screen.{width,height}.
• Element.offset{Top,Left,Bottom,Right}.
• Element.scroll{Width,Height}.

These return CSS pixels, and that means that in the current world of HiDPI screens and different screen resolutions, are all lossy.

Browsers do different things when converting these from fractional CSS pixels to an integer:

• Gecko rounds.
• WebKit (and Blink, inherited) truncate, unless you're very close to the next pixel, in which case they round. Which is a bit odd but hey.

It seems ideally we should have interop. I think some (if not all) of these (specially window sizes) would benefit from returning doubles, will file an issue on this. But it seems some agreement on what to do here would be on point.

Start by comparing the listed CSSOM View APIs with the Gecko, WebKit, and Blink conversion behaviors described in the issue. The work is done when the relevant specifications define an interoperable conversion rule, or resolve whether affected APIs should return doubles instead.