Have Frame::script() return a reference

Have Frame::script() return a reference instead of a pointer as it
can never return null. This makes the calling code look safer and
gets rid of a few unnecessary null-checks.

Review URL: https://codereview.chromium.org/33523002

git-svn-id: svn://svn.chromium.org/blink/trunk@160271 bbb929c8-8fbe-4397-9dbb-9b2b20218538