Support test harness (text-only) tests w/o expected results.

A large number of our existing text-only tests follow the convention of having JS assertions that either print "PASS", "FAIL", or "TIMEOUT" at the beginning of a line of text. If the test out therefor does not contain a FAIL or a TIMEOUT and contains at least one or more "PASS"es, it seems reasonable to assume that the test did actually pass (completely), and the -expected.txt is kinda redundant.

The testharness-based tests from the W3C follow this convention as well. Supporting this convention means that we don't need to review and check in thousands of boilerplate -expected.txt files, and we could potentially delete thousands of existing -expected.txt files in the Blink repo.

R=ojan@chromium.org, eseidel@chromium.org
BUG=268711

Review URL: https://codereview.chromium.org/22352002

git-svn-id: svn://svn.chromium.org/blink/trunk@159735 bbb929c8-8fbe-4397-9dbb-9b2b20218538