Chromium has a very useful network diagnostic tool called the NetLog and Cobalt is hooked up to use it. It's the main tool to track network traffic and debug network code.
The following command line switch will activate the NetLog and store net log record to the specified location.
./cobalt --net_log=/PATH/TO/YOUR_NETLOG_NAME.json The output json file will be stored at the file location you choose.
The produced json file is not human-friendly, use the NetLog Viewer
Cobalt's net_log can not enable some features in the web viewer, but all the network traffic is recorded in the event tab.
To Add NetLog entry, get the NetLog instance owned by NetworkModule to where you want to add entries and start/end your entry according to the NetLog interface.
A NetLog object is created at each NetworkModule initialization and is passed into Chromium net through URLRequestContext.