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If a STUN ping arrives before the remote description does, a prflx candidate will be created with an unknown generation. Once the remote description does arrive, the candidate's generation should be set so it can be sorted properly, and replaced by a non-prflx candidate once the candidate is signaled. BUG=webrtc:5752 R=honghaiz@webrtc.org, pthatcher@webrtc.org Review URL: https://codereview.webrtc.org/1868353004 . Cr-Commit-Position: refs/heads/master@{#12433}