Protect access to shared list of SRTP sessions.

This is a follow up to https://webrtc-codereview.appspot.com/47319004/
and locks access to the static list of SRTP sessions to prevent potential
race conditions.

BUG=4042
R=juberti@google.com

Review URL: https://webrtc-codereview.appspot.com/52609004

Cr-Commit-Position: refs/heads/master@{#9326}
1 file changed