Handle generated content with `display: block` correctly during flow
construction.

The iteration was incorrect here. Although it accidentally worked
before, it will cause problems when we have incremental style
recalculation.

The `after_block_iteration` reftest will become interesting once we have
incremental style recalc.
Merge pull request #3399 from pcwalton/fix-generated-content-iteration

Handle generated content with `display: block` correctly during flow