Protect access to shared list of SRTP sessions.
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9326}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e70028e43fded41310d41cc93b90f2e689c04725

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