This moves the code for threadjumping to get the RTP transport
despite its thread guard from the main function to two functions
marked especially "ForTesting".
Bug: webrtc:12230
Change-Id: I4473ed38e6fdedb05e2fbc97c2521bc1993fdd1d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/196521
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32792}
476859d38b