constellation: Stop using futures for frame sizes.

This will allow us to stop going to the DOM in order to handle iframe
sizing. Instead we can just store the pipeline and frame IDs of iframes
inside the flow tree itself.
auto merge of #1405 : pcwalton/servo/defuture, r=larsbergstrom

This will allow us to stop going to the DOM in order to handle iframe
sizing. Instead we can just store the pipeline and frame IDs of iframes
inside the flow tree itself.

r? @kmcallister