Fix public issue #39: Pick out the median runs based on CPU time,
not real time. Also, use nth_element instead of sort, since we
only need one element.

R=csilvers
DELTA=5  (3 added, 0 deleted, 2 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=1799


git-svn-id: https://snappy.googlecode.com/svn/trunk@35 03e5f5b5-db94-4691-08a0-1a8bf15f6143