Add a variety of DCHECK in the toy-server class to ensure that the backend is
non-null and remove the guard added in cr/246231018.

The underlying issue was a bug in the Chromium QuicSimplerServer which
is fixed in:

https://chromium-review.googlesource.com/c/chromium/src/+/1594063

gfe-relnote: n/a - Not used in the GFE
PiperOrigin-RevId: 246429041
Change-Id: I76103b0d992950fef950ba5bb8d508fe40ae5436
diff --git a/quic/tools/quic_server.cc b/quic/tools/quic_server.cc
index f8df59a..5542008 100644
--- a/quic/tools/quic_server.cc
+++ b/quic/tools/quic_server.cc
@@ -81,6 +81,7 @@
       packet_reader_(new QuicPacketReader()),
       quic_simple_server_backend_(quic_simple_server_backend),
       expected_connection_id_length_(expected_connection_id_length) {
+  DCHECK(quic_simple_server_backend_);
   Initialize();
 }