Simplify HTMLCollection predicates
auto merge of #705 : sonwow/servo/collection, r=jdm

Simplify some APIs that returns HTMLCollection in `dom::Document`.
In order to do this, `dom::htmldocument::createHTMLCollection` is moved to `dom::document::createHTMLCollection`.
Generate bindings for HTMLBRElement
Don't use a whole in-order traversal for computing heights.
auto merge of #692 : recrack/servo/brelement, r=jdm
Generate bindings for HTMLTableRowElement.
Generate bindings for HTMLTextAreaElement.
auto merge of #709 : jdm/servo/webidl, r=jdm
auto merge of #704 : eric93/servo/parallel-layout-ownedtree, r=pcwalton

Don't do a whole inorder traversal because we can't parallelize that. Parts of this traversal can be bottom-up
Generate bindings for HTMLUListElement
auto merge of #711 : jdm/servo/ulist, r=jdm
Generate bindings for HTMLTitleElement
auto merge of #710 : jdm/servo/titleelem, r=jdm
Generate bindings for HTMLCanvasElement.
Clean up importing.
auto merge of #713 : saneyuki/servo/canvas, r=jdm
add rect to FrameTree
add comment re: clipping rect
placeholders for FrameRectMsg from Layout to Constellation
add rect to FrameTree