Chrome Version: 68.0.3398.0

What steps will reproduce the problem?
(1) Run cluster telemetry leak detector for https://www.mkekabet.com
$ tools/perf/run_benchmark run leak_detection.cluster_telemetry --also-run-disabled-tests --user-agent=desktop --urls-list=http://www.mkekabet.com --archive-data-file=/Users/keishi/Downloads/59952/59952.json  --output-dir=/tmp --browser=release --device=desktop --pageset-repeat=1 --output-format=csv --show-stdout  -v
(2) final navigation to about:blank shows 404 error
(3) and a leak is detected

What is happening
(1) http://www.mkekabet.com uses cometd. cometd usually uses web sockets but when wpr proxy is used, web sockets don't work so it falls back to http long polling. (This is probably why the issue won't reproduce locally using the live site)
(2) Leak detector calls Page.Navigate("about:blank")
(3) http://www.mkekabet.com unload event handler runs, which issues an XHR to https://www.mkekabet.com/cometd/disconnect
(4) RenderFrameImpl::DidReceiveResponse gets the 404 response for https://www.mkekabet.com/cometd/disconnect and sets internal_data.http_status_code to 404
(5) Document::FinishedParsing for about:blank is called. It ends up calling RenderFrameImpl::RunScriptsAtDocumentReady which decides to show the 404 error page because (document_is_empty && internal_data.http_status_code is error).
According to the comments this is because, usually a blank document is likely to be a result of http error.
  // If this is an empty document with an http status code indicating an error,
  // we may want to display our own error page, so the user doesn't end up
  // with an unexplained blank page.