Fix three uninitialized members in rtp_receiver_impl.cc.
Fix three uninitialized members in rtp_receiver_impl.cc.

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1781004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 452d853c434d002f89a5c0ec5930f607fee05571

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