Add histograms to DnsProbeService

Measure how long probes take, what the results are, and compare to NCN's
offline state.

BUG=156415
TEST=none


Review URL: https://chromiumcodereview.appspot.com/11416019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@168942 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed