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

net/base/mapped_host_resolver.h

 // Copyright (c) 2011 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.
 
 #ifndef NET_BASE_MAPPED_HOST_RESOLVER_H_
 #define NET_BASE_MAPPED_HOST_RESOLVER_H_
 #pragma once
 
 #include <string>
 
@@ skipping 30 matching lines @@
   // Takes a comma separated list of rules, and assigns them to this resolver.
   void SetRulesFromString(const std::string& rules_string) {
     rules_.SetRulesFromString(rules_string);
   }
 
   // HostResolver methods:
   virtual int Resolve(const RequestInfo& info,
                       AddressList* addresses,
                       CompletionCallback* callback,
                       RequestHandle* out_req,
-                      const BoundNetLog& net_log);
-  virtual void CancelRequest(RequestHandle req);
-  virtual void AddObserver(Observer* observer);
-  virtual void RemoveObserver(Observer* observer);
-  virtual HostResolverImpl* GetAsHostResolverImpl();
+                      const BoundNetLog& net_log) OVERRIDE;
+  virtual int ResolveFromCache(const RequestInfo& info,
+                               AddressList* addresses,
+                               const BoundNetLog& net_log) OVERRIDE;
+  virtual void CancelRequest(RequestHandle req) OVERRIDE;
+  virtual void AddObserver(Observer* observer) OVERRIDE;
+  virtual void RemoveObserver(Observer* observer) OVERRIDE;
+  virtual HostResolverImpl* GetAsHostResolverImpl() OVERRIDE;
 
  private:
   scoped_ptr<HostResolver> impl_;
 
   HostMappingRules rules_;
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_MAPPED_HOST_RESOLVER_H_