Switch webkit_unit_tests to use the new test launcher.

BUG=290597
R=eseidel@chromium.org, phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/412683003

git-svn-id: svn://svn.chromium.org/blink/trunk@178798 bbb929c8-8fbe-4397-9dbb-9b2b20218538