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

base/callback.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $var MAX_ARITY = 6
 
 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
@@ skipping 256 matching lines @@
   template <typename T>
   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
       : CallbackBase(
           reinterpret_cast<InvokeFuncStorage>(&T::Invoker::DoInvoke),
           &invoker_holder.invoker_storage_) {
     COMPILE_ASSERT((is_same<PolymorphicInvoke,
                     typename T::Invoker::DoInvokeType>::value),
                    callback_type_does_not_match_bind_result);
   }
 
+  bool Equals(const Callback& other) const {
+    return CallbackBase::Equals(other);
+  }
+
   R Run($for ARG ,
         [[typename internal::ParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) const {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
     return f(invoker_storage_.get()[[]]
 $if ARITY != 0 [[, ]]
 $for ARG ,
              [[a$(ARG)]]);
   }
 };
 
 
 ]]  $$ for ARITY
 
 // Syntactic sugar to make Callbacks<void(void)> easier to declare since it
 // will be used in a lot of APIs with delayed execution.
 typedef Callback<void(void)> Closure;
 
 }  // namespace base
 
 #endif  // BASE_CALLBACK_H