Fix div by zero when no tests found in run-webkit-tests.

run-webkit-tests normally is robust against trying to run invalid or
missing tests, but it turns out that when run with the --run-chunk
argument there was a code path that tried to divide by zero because
we didn't err out early enough.

R=thestig@chromium.org
BUG=362749

Review URL: https://codereview.chromium.org/235863005

git-svn-id: svn://svn.chromium.org/blink/trunk@171395 bbb929c8-8fbe-4397-9dbb-9b2b20218538