Let's look at GetBrowserProtocolForChromeVersion:
https://github.com/BaristaLabs/chrome-dev-tools-generator/blob/master/src/BaristaLabs.ChromeDevTools.Core/Chrome.cs#L148
It uses following URL:
$"https://chromium.googlesource.com/chromium/src/+/{chromeVersion.WebKitVersionHash}/third_party/blink/renderer/core/inspector/browser_protocol.pdl?format=TEXT"
I have Chrome 66.0.3359.139.
{chromeVersion.WebKitVersionHash} is a020eddf0d85fe84d4a6787b304f50aafb670969 in this case.
It is really exist such tree:
https://chromium.googlesource.com/chromium/src/+/a020eddf0d85fe84d4a6787b304f50aafb670969/
Also it has "/thirdparty", "/thirdparty/blink"... but hasn't "/thirdparty/blink/renderer" and next folders.

Conclusion: not each googlesource tree has all required files. Some another way should be used.

Possible work-around:
Open root tree URL. Remember parent tree URL. If current tree hasn't such file - open parent tree URL. And so until file found. But I am hasn't tried it and it is obviously it could take many time.

P.S. I could contribute if you will merge :)