If you create a URL from a Windows file path (e.g., "C:/Some/Test/URL.html"), the Cache logic will eventually call the URL constructor that takes a ParsedURLStringTag argument. This constructor asserts that the URL created by the string matches the internal string representation. This turns out to be false on Windows, because the URL parsing logic skips the colon in the "C:/Some/Test" and changes it to "C/Some/Test", which does not match.
It looks like this happens in DumpRenderTree, but not MiniBrowser.
This issue has nothing to do with the URL implementation. It's a bug in a platform library.