Fix run-webkit-tests --reset-results crash.

Currently, None is being passed for expected_driver_output to
test_result_writer.write_test_result, but this causes 'NoneType'
errors when write_test_result uses expected_driver_output. This
can happen as a result of non-comparison failures, like timeouts.

This change makes write_test_result only call
create_repaint_overlay_result when expected_driver_output is
not None.

Review URL: https://codereview.chromium.org/636323002

git-svn-id: svn://svn.chromium.org/blink/trunk@183471 bbb929c8-8fbe-4397-9dbb-9b2b20218538