More minor improvements to BaseChannel/transport code. Mostly from late comments on this CL: https://codereview.webrtc.org/2614263002/ Changes SetTransport to DCHECK instead of returning false. Renames it to SetTransports. Fixes some possible transport resource leaks. BUG=None Review-Url: https://codereview.webrtc.org/2637503003 Cr-Original-Commit-Position: refs/heads/master@{#16130} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: bad5dadef3cf90d8ba081e7f814e913ecd2ff83d