Make the box-model layout-view work with ios

On firefox, we're using getBoundingClientRect + computed style to determine the width/height + margin/padding/border.

On chrome, we're using the `DOM.getBoxModel` command and then transform the returned quads in something that our layout-view can understand.

On ios, this command doesn't exist, so instead, we fall back to doing the same as on firefox:
- getting the bounding rect (by executing js code on the node, see https://github.com/campd/fxdt-adapters/wiki/Executing-random-code-on-a-DOM-node)
- getting the computed style (by using the `CSS.getComputedStyleForNode` command).