Bug 568863 implements a generic performance measurement utility that provides
back-end for linux. We want OSX and Windows back-ends as well.
This bug is for implementing the OSX back end. The code will need to be
written by someone deeply familiar with Darwin and its capabilities in this
area. You just need to code up js/src/perf/pm_osx.cpp and have the Makefile
build it - everything else is taken care of.