Only use PortAllocator on the network thread.

A previous CL started only using it on the worker thread, but it was
written before the network thread was introduced.

Review-Url: https://codereview.webrtc.org/1987093002
Cr-Original-Commit-Position: refs/heads/master@{#12802}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 91dd567feafdf832e8d0a47abec01def869d4f2b
3 files changed