Ensure that block elements with replaced content don't get their block size overwritten.
Merge pull request #3105 from glennw/block-replaced-content

Ensure that block elements with replaced content don't get their block s...
Prevent margin collapse in the root flow

According to the CSS specification, section 8.3.1, margins of the root
element's box do not collapse. Ensure that root flow margins do not
collapse during the assign heights phase.