Add a lock to NSSContext to fix data race

BUG=crbug/466784
R=juberti@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44669005

Cr-Original-Commit-Position: refs/heads/master@{#8871}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bef8d2d0208768d006e639701658ad4c9e73a69f
7 files changed