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

net/base/mapped_host_resolver.cc

Index: net/base/mapped_host_resolver.cc
diff --git a/net/base/mapped_host_resolver.cc b/net/base/mapped_host_resolver.cc
index 0e596f7f535fa4501253bc5eb00064477ec1d1b3..86912ffc8aefd0aae2917937a15f114a55bee988 100644
--- a/net/base/mapped_host_resolver.cc
+++ b/net/base/mapped_host_resolver.cc
@@ -7,6 +7,7 @@
 #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 {
@@ -23,6 +24,8 @@ int MappedHostResolver::Resolve(const RequestInfo& info,
                                 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());
@@ -31,6 +34,13 @@ int MappedHostResolver::Resolve(const RequestInfo& info,
   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);
 }