I believe no test exercises passes the is_validating check in QuicClientPromisedInfo::HandleClientRequest.  I added a DCHECK there and ran all net unittests matching *Quic*, and it was never hit.  It could be that filter missed something, or there's some disabled test that would have hit it if enabled.

I'm not sure how to trigger this case (One push stream that matches two requests, maybe?) and suspect that case may be related to issue 719461.  Regardless, we should have a test for this case, if it can happen.