Change print-json-test-results to use full_results.json by default.

Rather than always require the path to the json file to be passed,
we should support the normal set of default args and use the default
location for the layout test results.

R=eseidel@chromium.org
BUG=263930

Review URL: https://chromiumcodereview.appspot.com/20139002

git-svn-id: svn://svn.chromium.org/blink/trunk@154866 bbb929c8-8fbe-4397-9dbb-9b2b20218538