R=rmacnak@google.com Review-Url: https://codereview.chromium.org/2613283002 .
R=johnmccutchan@google.com Review URL: https://codereview.chromium.org/2480793002 .
Fuchsia provides pthreads so the linux implementation almost works except that fuchsia does not (currently) provide the clock_gettime clock for CLOCK_THREAD_CPUTIME_ID. Review URL: https://codereview.chromium.org/2154453002 .