Revert of Deletes mojo::Callback

mojo/public/cpp/bindings/lib/validation_errors.cc

 // Copyright 2014 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.
 
 #include "mojo/public/cpp/bindings/lib/validation_errors.h"
 
 #include "base/strings/stringprintf.h"
 #include "mojo/public/cpp/bindings/message.h"
 
 namespace mojo {
@@ skipping 71 matching lines @@
     if (context->message()) {
       context->message()->NotifyBadMessage(
           base::StringPrintf("Validation failed for %s [%s]",
                              context->description().data(),
                              ValidationErrorToString(error)));
     }
   }
 }
 
 ValidationErrorObserverForTesting::ValidationErrorObserverForTesting(
-    const base::Closure& callback)
+    const Callback<void()>& callback)
     : last_error_(VALIDATION_ERROR_NONE), callback_(callback) {
   DCHECK(!g_validation_error_observer);
   g_validation_error_observer = this;
 }
 
 ValidationErrorObserverForTesting::~ValidationErrorObserverForTesting() {
   DCHECK(g_validation_error_observer == this);
   g_validation_error_observer = nullptr;
 }
 
@@ skipping 13 matching lines @@
 }
 
 SerializationWarningObserverForTesting::
     ~SerializationWarningObserverForTesting() {
   DCHECK(g_serialization_warning_observer == this);
   g_serialization_warning_observer = nullptr;
 }
 
 }  // namespace internal
 }  // namespace mojo