Script streaming: fix streaming for empty scripts.

We never started the parsing task and never got the "parsing complete"
notification, so the client was just waiting forever for the script to finish.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/594653005

git-svn-id: svn://svn.chromium.org/blink/trunk@182484 bbb929c8-8fbe-4397-9dbb-9b2b20218538