[cssom-view] scroll properties and unscaled values

[mrego]

URL: https://drafts.csswg.org/cssom-view-1/#extension-to-the-element-interface

For several properties (e.g. clientWidth) it's mentioned they use the unscaled value, but that's not mentioned for scroll properties.

Apart from that there's no interop between how that works in Chromium/WebKit vs Firefox.
If you have an element where you do element.scrollTop = 100 and then you change zoom property to 1.5, in Chromium/WebKit element.scrollTop is still 100 but in Firefox it changes (see this Firefox's bug for more details).

CC @emilio