GTTF: Clean up TestResultsTracker, prepare for JSON summary output.

BUG=236893
R=sky@chromium.org

Review URL: https://codereview.chromium.org/26846006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228263 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228263