To repro assert failure:

$ third_party/WebKit/Tools/Scripts/run-webkit-tests -t gn_no_asan -v --additional-drt-flag=--site-per-process --no-retry-failures --additional-drt-flag=--no-sandbox --child-processes=3 --iterations=1 imported/wpt/html/syntax/parsing/html5lib_tests19.html

Callstack:

test_runner::TestRunner::tryToClearTopLoadingFrame()
blink::ProgressTracker::progressCompleted()
blink::FrameLoader::loadFailed()
blink::DocumentLoader::notifyFinished()
blink::Resource::checkNotify()
blink::ResourceFetcher::didFailLoading()
blink::ResourceLoader::didFail()
blink::ResourceLoader::cancel()
blink::ResourceFetcher::stopFetching()
blink::FrameLoader::stopAllLoaders()
blink::LocalFrame::detach()
blink::WebFrame::swap()
content::RenderFrameImpl::OnSwapOut()