Don't use a separate channel to communicate window size to the
constellation.

This will make sandboxing much easier since we won't need a
separate Unix pipe just for this.
file_loader: Handle file-not-found and other error conditions
Remove render() bypass for headless mode

render() isn't called in headless mode because the headless compositor never
sends a ReRenderMsg.

(This code was needed originally and that probably changed when CPU rendering
landed, but I'm not sure exactly why.)
auto merge of #1182 : pcwalton/servo/no-size-channel, r=metajack

This will make sandboxing much easier since we won't need a
separate Unix pipe just for this.
auto merge of #1195 : kmcallister/servo/headless, r=metajack
Parse `min-width` and `max-width`
Implement max-width, min-width for BlockFlow.