Script streaming: mem leak fix.

When streaming is suppressed and we didn't start the task, we were leaking it.

BUG=

Review URL: https://codereview.chromium.org/621843002

git-svn-id: svn://svn.chromium.org/blink/trunk@183063 bbb929c8-8fbe-4397-9dbb-9b2b20218538