Add a note to network::ProxyResolvingClientSocket on quic proxy

This CL adds a note to ProxyResolvingClientSocket on why QUIC proxy
isn't yet supported and links to crbug.

Bug: 876885
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I5d07064e252e8bdd9b28281488d88613fa102bd4
Reviewed-on: https://chromium-review.googlesource.com/1185746Reviewed-by: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585837}

services/network/proxy_resolving_client_socket.cc

@@ -264,8 +264,9 @@ int ProxyResolvingClientSocket::DoProxyResolveComplete(int result) {
   proxy_resolve_request_ = nullptr;
   if (result == net::OK) {
     // Removes unsupported proxies from the list. Currently, this removes
-    // just the SCHEME_QUIC proxy, which doesn't yet support tunneling.
-    // TODO(xunjieli): Allow QUIC proxy once it supports tunneling.
+    // just the SCHEME_QUIC proxy.
+    // TODO(crbug.com/876885): Allow QUIC proxy once net::QuicProxyClientSocket
+    // supports ReadIfReady() and CancelReadIfReady().
     proxy_info_.RemoveProxiesWithoutScheme(
         net::ProxyServer::SCHEME_DIRECT | net::ProxyServer::SCHEME_HTTP |
         net::ProxyServer::SCHEME_HTTPS | net::ProxyServer::SCHEME_SOCKS4 |