don't pretend to support dbus on windows in dbus_export.h

dbus/dbus_export.h

 // Copyright 2012 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 DBUS_DBUS_EXPORT_H_
 #define DBUS_DBUS_EXPORT_H_
 
 // Defines CHROME_DBUS_EXPORT so that functionality implemented by the dbus
 // library can be exported to consumers.
 // NOTE: We haven't used DBUS_EXPORT because it would conflict with the version
 // from /usr/include/dbus-1.0/dbus/dbus-macros.h.
 
+#if defined(WIN32)
+#error dbus support is not currently expected to work on windows
+#endif  // defined(WIN32)
+
 #if defined(COMPONENT_BUILD)
-#if defined(WIN32)
-
-#if defined(DBUS_IMPLEMENTATION)
-#define CHROME_DBUS_EXPORT __declspec(dllexport)
-#else
-#define CHROME_DBUS_EXPORT __declspec(dllimport)
-#endif  // defined(DBUS_IMPLEMENTATION)
-
-#else  // !defined(WIN32)
 
 #if defined(DBUS_IMPLEMENTATION)
 #define CHROME_DBUS_EXPORT __attribute__((visibility("default")))
 #else
 #define CHROME_DBUS_EXPORT
 #endif
 
-#endif  // defined(WIN32)
-
 #else  // !defined(COMPONENT_BUILD)
 
 #define CHROME_DBUS_EXPORT
 
 #endif  // defined(COMPONENT_BUILD)
 
 #endif  // DBUS_DBUS_EXPORT_H_