Chrome 49 added a highly useful event log which, among other things, contains the RTP packet headers of any packets received or sent. To capture it, expand the "Create Dump" section of chrome:https://webrtc-internals and click on the checkbox for "Enable diagnostic packet and event recording". This will create protobuf files starting with the selected base filename

<base filename>_<date>_<timestamp>_<render process ID>_<recording ID>

These files can be imported using either the rtc_event_log_visualizer provided by libwebrtc or this tool.

The native libwebrtc event log visualizer is more feature-complete but requires a WebRTC build environment since it is a C++ program which generates a python script.

This tool lets you import the dumps using just a webpage that processes data locally. It can also generate a pcap of the RTP/RTCP packets for inspection in Wireshark.

The proto file can be found in the webrtc source tree as either

logging/rtc_event_log/rtc_event_log2.proto

or rtc_event_log.proto.