Make RTPSender::SendPadData public.
Make RTPSender::SendPadData public.

R=andresp@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/5029004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c4726d06fa0553b4d673ecbbd632effc37e0f2e3

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