Lock access to ModuleRtpRtcpImpl::simulcast_.

Fixes race between RegisterSendPayload and SendOutgoingData.

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4099006

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5152 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed