auto merge of #1410 : pcwalton/servo/opaque-node, r=pcwalton

This shuts off all layout access to the DOM after flow tree construction, which will allow us to run script concurrently with layout during reflow and display list construction—even without the COW DOM. It also gets us closer to making `AbstractNode` noncopyable, which is important for safety because the JS GC will not trace layout.

r? @kmcallister
script: Make trees less generic