Fix whitespace_pre with incremental reflow turned on.

This implements fragment merging, in order to incrementally reflow linebroken
text. This makes the `whitespace_pre.html` reftest pass with incremental reflow
turned on with `-i`.
auto merge of #3689 : cgaebel/servo/working-incremental-flow-construction, r=pcwalton

This implements fragment merging, in order to incrementally reflow linebroken
text. This makes the `whitespace_pre.html` reftest pass with incremental reflow
turned on with `-i`.