fix swapped passes, failures in parsing layout test results

Also, make the step fail if the fulL_results.json file is missing, and
make the step warn if we get unexpected flakes or passes.

TBR=ilevy@chromium.org
BUG=293920

Review URL: https://codereview.chromium.org/26841003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227967 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227967