layout: Introduce a DOM leaf set as a prerequisite for parallel flow
construction.

This will be very slow until we have the concurrent hash table, but we
might as well get it in.
update per comment
auto merge of #1593 : pcwalton/servo/dom-leaf-set, r=metajack

construction.

This will be very slow until we have the concurrent hash table, but we
might as well get it in.

r? @metajack