With this change, if you supply the `-Z signpost` flag, Instruments.app
can display Servo events overlaid with callstack data. Even better, you
can get call stack profiling for individual Servo events (one layout,
one network request, one style recalculation, etc.)
This adds a dependency on the `signpost` crate. On unsupported OS's,
this crate is an no-op.
Screenshot: http://i.imgur.com/agLyzOU.png
r? @larsbergstrom
Source-Repo: https://github.com/servo/servo
Source-Revision: 7f84982de2fe8283d1df37d704842e037432087e
Patrick Walton
