Network service exposes socket functionalities via mojo APIs. 
One such case is network::ProxyResolvingSocketMojo which wraps around //net proxy socket classes.

With mojo sockets, we need a way to cancel pending reads without having buffered data in socket subclasses. This functionality is needed for upgrading a TCP socket to TLS. This requirement means that we need to use net::Socket::ReadIfReady() instead of net::Socket::Read().

I have implemented ReadIfReady() for HttpProxyClientSocket, SpdyProxyClientSocket and SOCKSClientSocket. To be consistent with other proxy socket classes, it will be good if QuicProxyClientSocket can support ReadIfReady as well.

Currently QuicProxyClientSocket is disabled in the mojo socket APIs (https://cs.chromium.org/chromium/src/services/network/proxy_resolving_client_socket.cc?rcl=0d326b48bd894605b4e41e22f6f085af7c12274b&l=269). I didn't find a way to add ReadIfReady() to QuicProxyClientSocket easily, so I am filing a bug for this one.

A side note:
Currently proxy_resolving_socket.mojom doesn't support upgrading to TLS. The other approach (if we want Quic proxies to be used for proxy_resolving_socket.mojom) is to modify services/network/socket_data_pump.cc to use Read() instead for network::ProxyResolvingSocketMojo and ReadIfReady() for network::TCPConnectedSocket, because only TCPConnectedSocket needs to support upgrading to TLS.