Minor cleanups in RTPSender::UpdateRtpStats
Minor cleanups in RTPSender::UpdateRtpStats

ssrc taken from packet instead of module removing extra lock
removed unneccesary call to clock_
reduced number of lines.

BUG=webrtc:5565
R=brandtr@webrtc.org

Review URL: https://codereview.webrtc.org/2352023002 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14307}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d69e526440eed24b8b2977130500f7c4e6592be7

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk