net: Quiet QuicSimpleServer's shutdown warning to reduce test logspam

If this log line isn't needed in client builds, WDYT about changing it
to a DVLOG?

It's pretty noisy right now; for instance, see this example run
from URLLoader tests: https://pastebin.com/7YBwscgP.

Change-Id: Ica12950cf9149a3027148c532e29a32ff28798ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298639Reviewed-by: David Schinazi <dschinazi@chromium.org>
Commit-Queue: David Schinazi <dschinazi@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788440}

net/tools/quic/quic_simple_server.cc

@@ -125,7 +125,7 @@ bool QuicSimpleServer::Listen(const IPEndPoint& address) {
 }
 
 void QuicSimpleServer::Shutdown() {
-  LOG(WARNING) << "QuicSimpleServer is shutting down";
+  DVLOG(1) << "QuicSimpleServer is shutting down";
   // Before we shut down the epoll server, give all active sessions a chance to
   // notify clients that they're closing.
   dispatcher_->Shutdown();