Move build_element_from_tag out of the HTML parser.

This function is not particular to the parser, so should live in the DOM.
auto merge of #3853 : Ms2ger/servo/create, r=Manishearth

This function is not particular to the parser, so should live in the DOM.
Have ContentBox(es)Queries consult the flow tree

Instead of looking at the display tree, have ContentBox(es)Query consult
the flow tree. This allow optimizing away parts of the display tree
later. To do this we need to be more careful about how we send reflow
requests, only querying the flow tree when possible.

Fixes #3790.