system_wrapper/Atomic32 has been deprecated (which is already just a
wrapper of std::atomic) in favor of platform-independent std::atomic
from C++11. This CL replaces all use of Atomic32 in video/
Bug: webrtc:8428
Change-Id: If4dab4909df06944c009e7b70141f58daef7be10
Reviewed-on: https://webrtc-review.googlesource.com/14720
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Yuwei Huang <yuweih@google.com>
Cr-Commit-Position: refs/heads/master@{#20417}