layout: Implement parallel reflow for the bubble-widths and
assign-heights phases.

This uses the new work-stealing deque. By default, 3/4 of a thread per
logical CPU is used. This can be tuned with the `-y` flag.

I measured a 65% reflow speedup on `perf-rainbow.html` and a 247% reflow
speedup on `http://en.wikipedia.org/wiki/South_China_Sea` on a 4-core
HyperThreaded Core i7. However, numbers were fairly volatile, especially
for the latter.
auto merge of #1493 : pcwalton/servo/parallel-layout-new, r=pcwalton,metajack

r? @metajack
Implement white-space property(pre)

In order to support line-break by new-line character,
Fist, calculate new-line character's position.(fn flush_clump_to_list)
Second, split box(which includes new-line character) and do line-break.(fn scan_for_lines)