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
8 files changed