layout: Make `NodeRange` use `OpaqueNode` for memory safety.

These were stored in inline flows. This was caught by my WIP changes to
harden layout (yay for Rust!)
auto merge of #1426 : pcwalton/servo/opaque-node-range, r=larsbergstrom

These were stored in inline flows. This was caught by my WIP changes to
harden layout (yay for Rust!)

r? @larsbergstrom