This CL adds a lock to stashed_images_ in MultiplexEncoderAdapter. Without lock, it is possible that different threads acts on stashed_images_ simultaneously and leads to crash. Bug: webrtc:8965 Change-Id: I887861092d185c3bd6047eb529d8c1cf57fa4648 Reviewed-on: https://webrtc-review.googlesource.com/59260 Reviewed-by: Emircan Uysaler <emircan@webrtc.org> Commit-Queue: Qiang Chen <qiangchen@chromium.org> Cr-Commit-Position: refs/heads/master@{#22261}