Telemetry: Check in PrintPerfResult that the flattened list is also non-zero.

BUG=304335

Review URL: https://codereview.chromium.org/25999004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227271 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227271