[cssom-view] Define rounding for clientWidth & friends

[Loirooriol]

<div id="target" style="width: 9.9px"></div>

	Gecko	Blink & WebKit	Servo
target.getBoundingClientRect().width	9.899993896484375	9.890625	9.9
target.clientWidth	10	10	9   10

The definition of clientWidth is

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

readonly attribute long clientWidth;

https://drafts.csswg.org/cssom-view/#dom-element-clientwidth

Return the width of the padding edge excluding the width of any rendered scrollbar between the padding edge and the border edge, ignoring any transforms that apply to the element and its ancestors.

It doesn't mention any kind of rounding, I'm assuming this is handled by https://webidl.spec.whatwg.org/#js-long
If I'm getting it right, it's supposed to just truncate the number, like Servo used to do, but that's not what the other browser are doing.