Fix data race in AMP.
Fix data race in AMP.

R=aluebs@webrtc.org, aluebs-webrtc
BUG=516637

Review URL: https://codereview.webrtc.org/1279953003 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9694}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4bc66fc3879e7bd3fcd6f7dedf0c4bb35e88e66f

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc