Surface ERR_SPDY_SESSION_ALREADY_EXISTS to the proxy delegate.

net/http/http_proxy_client_socket_pool.cc

Index: net/http/http_proxy_client_socket_pool.cc
diff --git a/net/http/http_proxy_client_socket_pool.cc b/net/http/http_proxy_client_socket_pool.cc
index 37ffa6d1a436567d24cdbe7ffb558c9c81d96f3b..829a5e78314968bca7b6acce7b9f6fd3c7215482 100644
--- a/net/http/http_proxy_client_socket_pool.cc
+++ b/net/http/http_proxy_client_socket_pool.cc
@@ -237,6 +237,13 @@ int HttpProxyConnectJob::DoSSLConnectComplete(int result) {
       return ERR_PROXY_CERTIFICATE_INVALID;
     }
   }
+  // A SPDY session to the proxy completed prior to resolving the proxy
+  // hostname. Surface this error, and allow the delegate to retry.
+  // See crbug.com/334413.
+  if (result == ERR_SPDY_SESSION_ALREADY_EXISTS) {
+    DCHECK(!transport_socket_handle_->socket());
+    return ERR_SPDY_SESSION_ALREADY_EXISTS;
+  }
   if (result < 0) {
     if (transport_socket_handle_->socket())
       transport_socket_handle_->socket()->Disconnect();