The RTCStatsCollector collects candidates from candidate pairs. Note
that there may be other candidates that are not paired with anything,
stats for these should also be produced before closing crbug.com/632723.
[1] https://w3c.github.io/webrtc-stats/#icecandidate-dict*
BUG=chromium:627816, chromium:632723
Review-Url: https://codereview.webrtc.org/2384143002
Cr-Commit-Position: refs/heads/master@{#14565}