HostResolver: don't interpret NULL callback argument as a request to do synchronous resolution.

net/base/mapped_host_resolver.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/base/mapped_host_resolver.h"
 
 #include "base/string_tokenizer.h"
 #include "base/string_util.h"
 #include "net/base/host_port_pair.h"
+#include "net/base/net_errors.h"
 #include "net/base/net_util.h"
 
 namespace net {
 
 MappedHostResolver::MappedHostResolver(HostResolver* impl)
     : impl_(impl) {
 }
 
 MappedHostResolver::~MappedHostResolver() {
 }
 
 int MappedHostResolver::Resolve(const RequestInfo& info,
                                 AddressList* addresses,
                                 CompletionCallback* callback,
                                 RequestHandle* out_req,
                                 const BoundNetLog& net_log) {
+  DCHECK(addresses);
+  DCHECK(callback);
   // Modify the request before forwarding it to |impl_|.
   RequestInfo modified_info = info;
   HostPortPair host_port(info.host_port_pair());
   if (rules_.RewriteHost(&host_port))
     modified_info.set_host_port_pair(host_port);
   return impl_->Resolve(modified_info, addresses, callback, out_req, net_log);
 }
 
+int MappedHostResolver::ResolveFromCache(const RequestInfo& info,
+                                         AddressList* addresses,
+                                         const BoundNetLog& net_log) {
+  NOTIMPLEMENTED();
+  return ERR_UNEXPECTED;
+}
+
 void MappedHostResolver::CancelRequest(RequestHandle req) {
   impl_->CancelRequest(req);
 }
 
 void MappedHostResolver::AddObserver(Observer* observer) {
   impl_->AddObserver(observer);
 }
 
 void MappedHostResolver::RemoveObserver(Observer* observer) {
   impl_->RemoveObserver(observer);
 }
 
 HostResolverImpl* MappedHostResolver::GetAsHostResolverImpl() {
   return impl_->GetAsHostResolverImpl();
 }
 
 }  // namespace net