Make Callback.Equals() only work when comparing against the same type.

base/callback_internal.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.
 
 // This file contains utility functions and classes that help the
 // implementation, and management of the Callback objects.
 
 #ifndef BASE_CALLBACK_INTERNAL_H_
 #define BASE_CALLBACK_INTERNAL_H_
 #pragma once
@@ skipping 44 matching lines @@
 // Holds the Callback methods that don't require specialization to reduce
 // template bloat.
 class BASE_EXPORT CallbackBase {
  public:
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const;
 
   // Returns the Callback into an uninitalized state.
   void Reset();
 
+ protected:
   bool Equals(const CallbackBase& other) const;
 
- protected:
   // In C++, it is safe to cast function pointers to function pointers of
   // another type. It is not okay to use void*. We create a InvokeFuncStorage
   // that that can store our function pointer, and then cast it back to
   // the original type on usage.
   typedef void(*InvokeFuncStorage)(void);
 
   CallbackBase(InvokeFuncStorage polymorphic_invoke,
                scoped_refptr<InvokerStorageBase>* invoker_storage);
 
   // Force the destructor to be instaniated inside this translation unit so
@@ skipping 48 matching lines @@
 template <typename T>
 struct ParamTraits<T[]> {
   typedef const T* ForwardType;
   typedef const T* StorageType;
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_