layout: Change `~Box` to `Box`.

63% improvement in box building on the rainbow page.
auto merge of #1371 : pcwalton/servo/flatten-box, r=larsbergstrom

63% improvement in box building on the rainbow page.

r? @larsbergstrom
make font/platform/gfx/font_context safe
remove @ in LayoutTask.FontContext
makes linux/Arc<FontContextHandle> Rc<FontContextHandle>
auto merge of #1372 : ksh8281/servo/remove_@_in_LayoutTask.FontContext2, r=pcwalton

Remove @ in layout task.font_context & fix linux/FontContextHandle
Arc<FontContextHandle> -> Rc<FontContextHandle>