Fix race condition for GetContributingSources test.
Fix race condition for GetContributingSources test.

Bug: webrtc:9813
Change-Id: I44f50f9858217c8303862f3820db11dbd8736b6c
Reviewed-on: https://webrtc-review.googlesource.com/c/104121
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Reviewed-by: Jonas Oreland <d14c41fb00799e631b5abdf9ef3f616c4906b209@webrtc.org>
Commit-Queue: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25042}
TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc