Script streaming fix: don't ref() ScriptStreamer too early.

If we end up suppressing the streaming before the background task is created,
the corresponding deref() is never called.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/602543002

git-svn-id: svn://svn.chromium.org/blink/trunk@182587 bbb929c8-8fbe-4397-9dbb-9b2b20218538