This commit puts floats behind the `layout.floats.enabled` pref, because of the
following issues and unimplemented features:

* Inline formatting contexts don't take floats into account, so text doesn't
  flow around the floats yet.

* Non-floated block formatting contexts don't take floats into account, so BFCs
  can overlap floats.

* The `clear` property is only respected on floats, not on other elements; i.e.
  regular boxes can't clear past floats yet.

* Block formatting contexts that contain floats don't expand vertically to
  contain all the floats. That is, floats can stick out the bottom of BFCs,
  contra spec.

* Floats don't respect negative margins yet.

* Floats are painted in the same order as regular content, contra spec.