Propogate network-worker thread split to api

BUG=webrtc:5645

Review-Url: https://codereview.webrtc.org/1968393002
Cr-Original-Commit-Position: refs/heads/master@{#12767}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e9021a3601eff3e33a103ae35fb866554ee28747