Fix white-space property(pre)

If html has empty line, it is not handled currently.
Because pending line is empty in such case, flushing current line doesn't care empty line.
For example, below html has two empty line(line 2 and line 4)
<div>
test1

test2

</div>

To handle empty line, added empty box to pending line.
auto merge of #1547 : deokjinkim/servo/fix_white_space, r=metajack

If html has empty line, it is not handled currently.
Because pending line is empty in such case, flushing current line doesn't care empty line.
For example, below html has two empty line(line 2 and line 4)
<div>
test1

test2

</div>

To handle empty line, added empty box to pending line.