Revert of Deletes mojo::Callback

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

 // 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_VALIDATION_ERRORS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_VALIDATION_ERRORS_H_
 
-#include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
+#include "mojo/public/cpp/bindings/callback.h"
 #include "mojo/public/cpp/bindings/lib/validation_context.h"
 
 namespace mojo {
 namespace internal {
 
 enum ValidationError {
   // There is no validation error.
   VALIDATION_ERROR_NONE,
   // An object (struct or array) is not 8-byte aligned.
   VALIDATION_ERROR_MISALIGNED_OBJECT,
@@ skipping 51 matching lines @@
 const char* ValidationErrorToString(ValidationError error);
 
 void ReportValidationError(ValidationContext* context,
                            ValidationError error,
                            const char* description = nullptr);
 
 // Only used by validation tests and when there is only one thread doing message
 // validation.
 class ValidationErrorObserverForTesting {
  public:
-  explicit ValidationErrorObserverForTesting(const base::Closure& callback);
+  explicit ValidationErrorObserverForTesting(const Callback<void()>& callback);
   ~ValidationErrorObserverForTesting();
 
   ValidationError last_error() const { return last_error_; }
   void set_last_error(ValidationError error) {
     last_error_ = error;
     callback_.Run();
   }
 
  private:
   ValidationError last_error_;
-  base::Closure callback_;
+  Callback<void()> callback_;
 
   DISALLOW_COPY_AND_ASSIGN(ValidationErrorObserverForTesting);
 };
 
 // Used only by MOJO_INTERNAL_DLOG_SERIALIZATION_WARNING. Don't use it directly.
 //
 // The function returns true if the error is recorded (by a
 // SerializationWarningObserverForTesting object), false otherwise.
 bool ReportSerializationWarning(ValidationError error);
 
@@ skipping 26 matching lines @@
 //
 // In non-debug build, does nothing (not even compiling |condition|).
 #define MOJO_INTERNAL_DLOG_SERIALIZATION_WARNING(condition, error,    \
                                                  description)         \
   DLOG_IF(FATAL, (condition) && !ReportSerializationWarning(error))   \
       << "The outgoing message will trigger "                         \
       << ValidationErrorToString(error) << " at the receiving side (" \
       << description << ").";
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_VALIDATION_ERRORS_H_