socket: Force tunneling for all sockets

After the upstream large refactor, now only WebSocket sockets
have tunneling via HTTP/1 proxies. "Raw" sockets in the normal
socket pool don't have tunneling via HTTP/1 proxies, i.e.
CONNECT headers are not sent, instead the raw payload is sent
as-is to the HTTP/1 proxy, which makes it not work.

For the reference the official code does this:

- HTTP sockets via HTTP/1 proxies: normal pool, no tunneling.
- HTTPS sockets via HTTP/1 proxies: normal pool, no tunneling
  but does its own proxy encapsulation.
- WS sockets via HTTP/1 proxies: WS pool, tunneling.

We want the normal pool because the WS pool has some extra
restrictions but we also want tunneling to expose a client socket
with the proxy built in.

Therefore we can force tunneling for all sockets. This will always
send CONNECT headers first and thus break HTTP client sockets via
HTTP/1 proxies, but since we don't use this combination this is ok.

src/net/socket/client_socket_pool_manager_impl.cc

@@ -85,7 +85,7 @@ ClientSocketPool* ClientSocketPoolManagerImpl::GetSocketPool(
     new_pool = std::make_unique<TransportClientSocketPool>(
         sockets_per_proxy_server, sockets_per_group,
         unused_idle_socket_timeout(pool_type_), proxy_server,
-        pool_type_ == HttpNetworkSession::WEBSOCKET_SOCKET_POOL,
+        true /* force_tunnel */,
         &common_connect_job_params_, ssl_config_service_);
   }