Disable Alt-Svc in Crnet.

Alt-Svc is currently disabled in Cronet (in
components/cronet/android/*url_request_context_adapter.cc), until this feature
is thorougly tested in Chromium.  It should also be disabled in Crnet.

Review URL: https://codereview.chromium.org/1643463004

Cr-Commit-Position: refs/heads/master@{#371903}

ios/crnet/crnet_environment.mm

@@ -438,7 +438,7 @@ void CrNetEnvironment::InitializeOnNetworkThread() {
   params.net_log = main_context_->net_log();
   params.next_protos =
       net::NextProtosWithSpdyAndQuic(spdy_enabled(), quic_enabled());
-  params.use_alternative_services = true;
+  params.use_alternative_services = false;
   params.enable_quic = quic_enabled();
   params.alternative_service_probability_threshold =
       alternate_protocol_threshold_;