The protocol.json has split into separate files. https://crbug.com/580337

One for the browser, one for a V8 javascript environment:
    src/third_party/WebKit/Source/core/inspector/browser_protocol.json
    src/v8/src/inspector/js_protocol.json

Browsable here:
    https://chromium.googlesource.com/chromium/src/+/master/third_party/WebKit/Source/core/inspector/browser_protocol.json
    https://chromium.googlesource.com/v8/v8/+/master/src/inspector/js_protocol.json

More details on the protocol:
    https://developer.chrome.com/devtools/docs/debugger-protocol