[CSS Regions] Fix selection highlight

Implement subtrees approach explained at:
https://github.com/Igalia/css-regions-selection/wiki/Selection-&-CSS-Regions

Tested the following use cases successfully:
* http://igalia.github.io/css-regions-selection/
* http://people.igalia.com/mrego/css-regions/

TODO:
* Adapt RenderView::selectionBounds() to the subtrees approach.