Lock access to ModuleRtpRtcpImpl::simulcast_.
Lock access to ModuleRtpRtcpImpl::simulcast_.

Fixes race between RegisterSendPayload and SendOutgoingData.

BUG=
R=stefan@webrtc.org

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

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ffe1b17b57a6f617f33a85ff43905a145b4fed92

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