Suppress memory leak report from MetricsNetworkObserver::ProbeWifiPHYLayerProtocol
TBR=asvitkine@chromium.org
BUG=328987
TEST=waterfall

Review URL: https://codereview.chromium.org/112703004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241018 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241018