style: Short-circuit the selector matching process if the map is empty.

125% win on selector matching (!)
style: Stop allocating when lowercasing element names for lookup in the
hash.

31% win on selector matching.
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 #1574 : pcwalton/servo/cheat-before-after, r=SimonSapin

125% win on selector matching (!)

r? @SimonSapin
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.
auto merge of #1566 : pcwalton/servo/fewer-allocations, r=SimonSapin

hash.

31% win on selector matching.

r? @larsbergstrom