Fix tiny race condition when Vp9NonFlexMode_* tests were failing.
Fix tiny race condition when Vp9NonFlexMode_* tests were failing.

TSAN complained at each failure, which only added confusion.

Bug: webrtc:11243
Change-Id: Ic36ec13562699478be23050a7b723e1b9bb47f0e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/164528
Reviewed-by: Henrik Andreassson <e468f06af92d21183a2a635f545ff5cefc8c8b65@webrtc.org>
Commit-Queue: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@google.com>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30161}

TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc