Telemetry: Print number of failed and errored pages.

Print the number of failed and errored pages as a new metric. This will help
track the performance of flaky tests over time.

BUG=238864

Review URL: https://codereview.chromium.org/25275003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226313 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226313