[cssom-view] should define rounding better

[dbaron]

Many definitions in cssom-view (for example, the definition of offsetWidth) should describe the rounding that needs to be performed. To take offsetWidth as an example:

• the property is a long
• the spec's definition says:

  Return the border edge width of the first CSS layout box associated with the element, ignoring any transforms that apply to the element and its ancestors.

• the thing described by the spec as the result is often not an integer. (For that matter, the spec doesn't even say what units to use for the width!)

The spec ought to describe what sort of rounding is performed (round-to-nearest, floor, etc.), likely through the use of terms that could be reused elsewhere (see, say, whatwg/html#4072, which led me to file this).