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

net/base/mapped_host_resolver.h

Index: net/base/mapped_host_resolver.h
diff --git a/net/base/mapped_host_resolver.h b/net/base/mapped_host_resolver.h
index 343c5e1545472c2017f3a2679beea8e9724581f8..a589fa370716f42a53afe9f6c458efb6ace781b1 100644
--- a/net/base/mapped_host_resolver.h
+++ b/net/base/mapped_host_resolver.h
@@ -48,11 +48,14 @@ class NET_API MappedHostResolver : public HostResolver {
                       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_;