Fix a bug in RtpFileSource related to RTCP packets in rtpdump files
Fix a bug in RtpFileSource related to RTCP packets in rtpdump files

According to http://www.cs.columbia.edu/irt/software/rtptools/#rtpdump,
RTCP packets are marked with plen==0. In this class, plen is mapped to
original_length, not length.

Review URL: https://codereview.webrtc.org/1356543002

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9981}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 11d583f41484913fd1e7b3e283966eb7b7e11ed2

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