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.
I don't think we can eliminate it yet. Hit box and mouse over testing is still done using the display list and I think a flow tree implementation is more difficult, because of z-index considerations. The paint order is encoded more readily in the display list than the flow tree, though admittedly I don't understand the mechanism ATM.