Script streaming: fix small script recognition heuristic.

It can happen that the first data chunk for a large script is small. To handle
this, we don't give up with streaming immediately when we receive a small data
chunk, but simply delay the decision to start the streaming.

See crbug.com/425475 for discussion about when this can happen.

BUG=

Review URL: https://codereview.chromium.org/655263003

git-svn-id: svn://svn.chromium.org/blink/trunk@184201 bbb929c8-8fbe-4397-9dbb-9b2b20218538