Reset ssrc when calling SetSendCodec.
Reset ssrc when calling SetSendCodec.

BUG=1398
TEST=Tested locally.

Review URL: https://webrtc-codereview.appspot.com/1107004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 294e5b0b82ae181245cbaedbe3542529214d408e

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