Add some basic debug info for dumping flows.
auto merge of #1212 : larsbergstrom/servo/dump_flows, r=brson

With @ryanhc

Will add more debugging information for RenderBoxes in InlineFlows and `dump_str` for each of the RenderBoxes later.
auto merge of #1206 : Ms2ger/servo/strings, r=jdm

I'll probably do a followup to actually pass non-nullable strings to the native functions.
Make DOMString represent a non-nullable string.
Don't pass nullable strings to native DOM methods that want non-nullable strings. Fixes #1207.