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

net/base/mock_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_MOCK_HOST_RESOLVER_H_
 #define NET_BASE_MOCK_HOST_RESOLVER_H_
 #pragma once
 
 #include <list>
 
@@ skipping 45 matching lines @@
                           size_t max_pending_requests) {
     impl_->SetPoolConstraints(
         pool_index, max_outstanding_jobs, max_pending_requests);
   }
 
   // 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);
+                      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;
 
  protected:
   MockHostResolverBase(bool use_caching);
 
   scoped_ptr<HostResolverImpl> impl_;
   scoped_refptr<RuleBasedHostResolverProc> rules_;
   bool synchronous_mode_;
   bool use_caching_;
 
  private:
@@ skipping 110 matching lines @@
   void Init(HostResolverProc* proc);
 
  private:
   scoped_refptr<HostResolverProc> current_proc_;
   scoped_refptr<HostResolverProc> previous_proc_;
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_MOCK_HOST_RESOLVER_H_