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/blink/renderer/core/inspector/browser_protocol.pdl
    src/v8/src/inspector/js_protocol.pdl

Browsable here:
    https://chromium.googlesource.com/chromium/src/+/master/third_party/blink/renderer/core/inspector/browser_protocol.pdl
    https://chromium.googlesource.com/v8/v8/+/master/src/inspector/js_protocol.pdl

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