Remove RtpPacketToSend::GetHeader as almost unused. Merge rtp::Packet::GetHeader into RtpPacketReceived::GetHeader removing error-prone code where latter shadow former version BUG=None Review-Url: https://codereview.webrtc.org/3012983002 Cr-Original-Commit-Position: refs/heads/master@{#19784} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: ce25181714243770e4eff3a636ae8d49eca6d92b