Adding a critical section missing in r5543.
Adding a critical section missing in r5543.

This fixes a race caught by the linux tsan bot.

R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/8739004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9075d519a2317ba2318aa855112396f968311fc4

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