Stop cloning FrameTrees

This fixes an issue where a ChildFrameTree is inserted into a clone of its
parent, rather than its "real" parent, as noted in #2124.
auto merge of #2134 : mbrubeck/servo/frametree-clone, r=larsbergstrom

This fixes an issue where a ChildFrameTree is inserted into a clone of its parent, rather than its "real" parent, as noted in #2124.

This moves `FrameTree::iter` and the methods that depend on it into a new trait that is implemented on `Rc<FrameTree>`, since it's safe to clone an `Rc<FrameTree>`.

r? @larsbergstrom