To simplify things, the candidate pool is only used in the first offer/answer. After setting a local description, the size is frozen, and changing ICE servers won't refresh the pool. After setting an answer, the pooled candidates are discarded. BUG=webrtc:5180 Review-Url: https://codereview.webrtc.org/2717893003 Cr-Commit-Position: refs/heads/master@{#17178}