Replace fprintf with mojo logging macro.

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 <assert.h>
-#include <stdio.h>
+
+#include "mojo/public/cpp/environment/logging.h"
 
 namespace mojo {
 namespace internal {
 namespace {
 
 ValidationErrorObserverForTesting* g_validation_error_observer = NULL;
 
 }  // namespace
 
 const char* ValidationErrorToString(ValidationError error) {
@@ skipping 15 matching lines @@
     case VALIDATION_ERROR_MESSAGE_HEADER_INVALID_FLAG_COMBINATION:
       return "VALIDATION_ERROR_MESSAGE_HEADER_INVALID_FLAG_COMBINATION";
     case VALIDATION_ERROR_MESSAGE_HEADER_MISSING_REQUEST_ID:
       return "VALIDATION_ERROR_MESSAGE_HEADER_MISSING_REQUEST_ID";
   }
 
   return "Unknown error";
 }
 
 void ReportValidationError(ValidationError error) {
-  if (g_validation_error_observer) {
+  if (g_validation_error_observer)
     g_validation_error_observer->set_last_error(error);
-  } else {
-    // TODO(yzshen): Consider adding better logging support.
-    fprintf(stderr, "Invalid message: %s\n", ValidationErrorToString(error));
-  }
+  else
+    MOJO_LOG(ERROR) << "Invalid message: " << ValidationErrorToString(error);
 }
 
 ValidationErrorObserverForTesting::ValidationErrorObserverForTesting()
     : last_error_(VALIDATION_ERROR_NONE) {
   assert(!g_validation_error_observer);
   g_validation_error_observer = this;
 }
 
 ValidationErrorObserverForTesting::~ValidationErrorObserverForTesting() {
   assert(g_validation_error_observer == this);
   g_validation_error_observer = NULL;
 }
 
 }  // namespace internal
 }  // namespace mojo