Tighten link-local routing exclusion check

Also add a unit test for this behavior.

BUG=https://code.google.com/p/webrtc/issues/detail?id=4823
R=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1218293016 .

Cr-Commit-Position: refs/heads/master@{#9550}