Using explicit atomic operations permits TSan to understand them and prevents false positives. Downgrading the atomic Load to acquire semantics. This reduces the number of memory barriers inserted from two down to one at most. Also renaming Load/Store to AcquireLoad/ReleaseStore. BUG=chromium:512382 R=dvyukov@chromium.org, glider@chromium.org TBR=tommi@webrtc.org Review URL: https://codereview.webrtc.org/1246073002 Cr-Commit-Position: refs/heads/master@{#9613}