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
6 files changed