Add a PeerConnection integration test for adding an audio track mid-call

BUG=None

Review-Url: https://codereview.webrtc.org/3007923002
Cr-Original-Commit-Position: refs/heads/master@{#19731}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4389b4d96bb98cf210b761b4f152434fceb767a6
1 file changed