Better determination of Symmetric NAT.

If we're using shared socket mode, Symmetric NAT can be correctly determined by having more than 1 srflx per Requester (i.e. socket).

Design Doc updated at https://docs.google.com/document/d/11zruojoNqZgZYDrSVk0O3JHqFz2cFldWYQbroZdZRBg/edit#heading=h.t7npdccia97t

BUG=4576
R=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1166013002

Cr-Original-Commit-Position: refs/heads/master@{#9396}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 72e9f044471078f40999c93823424e4f0e825c21
4 files changed