Remove unintentional copies due to RefCell<T>::get

We are calling `get` when we should call `borrow` or `borrow_mut`.  This is
causing bugs with iframes because we append to a copy of the `children` vector
rather than the original.
auto merge of #2130 : mbrubeck/servo/frametree-copy-fix, r=larsbergstrom

We are calling `get` when we should call `borrow` or `borrow_mut`.  This is causing bugs with iframes because we append the child frame to a copy of the parent's `children` vector rather than the original.

r? @larsbergstrom