How to save websocket frames in Chrome

From Chrome 76 the HAR file now includes WebSocket messages.

WebSocket messages in HAR exports

The _webSocketMessages property begins with an underscore to indicate that it’s a custom field.

...
"_webSocketMessages": [
  {
    "type": "send",
    "time": 1558730482.5071473,
    "opcode": 1,
    "data": "Hello, WebSockets!"
  },
  {
    "type": "receive",
    "time": 1558730482.5883863,
    "opcode": 1,
    "data": "Hello, WebSockets!"
  }
]
...