layout: Reference count `ComputedValues` structures like Gecko does.

This has no difference in CSS selector matching performance and results
in a 31% speedup in constraint solving on the rainbow page.
auto merge of #1387 : pcwalton/servo/ref-count-style, r=pcwalton

This has no difference in CSS selector matching performance and results
in a 31% speedup in constraint solving on the rainbow page.

r? @kmcallister