Add NodeHelpers::get_bounding_content_box().
Add NodeHelpers::get_content_boxes().
Add Page::hit_test().
Add Page::get_nodes_under_mouse().
auto merge of #2210 : saneyuki/servo/query, r=jdm

- see #356
- To decrease the conversion code, `JS<Node>` -> `Node`, I added API methods to `NodeHelpers`.
- I added APIs to `Page` not `Document` because I had thought that their APIs returns the hitted nodes address in the "page".