Restore consistency of thread names in ThreadController and P2PTransportChannel. This is a follow-up for https://codereview.webrtc.org/1895813003 and https://codereview.webrtc.org/1903393004. BUG=webrtc:6432 Review-Url: https://codereview.webrtc.org/2378573003 Cr-Commit-Position: refs/heads/master@{#14426}