Once CSSOM View defines "client window", use that for all window manipulation

[gsnedders]

w3c/csswg-drafts#430 is open for defining it.

From there, e.g.,

Ideally, WebDriver could just say, "Set the width, in CSS pixels, of the client window to a value as close as possible to to width". That requires some citable definition of "client window", however.

[gsnedders]

Note also that we should define tie breaking here.