Remove port.worker_startup_delay from run-webkit-tests / webkitpy.

Now that we wait for #READY from content_shell, we no longer need an
arbitary between starting up each worker.

R=ojan@chromum.org
BUG=

Review URL: https://codereview.chromium.org/55133007

git-svn-id: svn://svn.chromium.org/blink/trunk@161107 bbb929c8-8fbe-4397-9dbb-9b2b20218538