components: Cleanup class/struct fwd declarations

components/storage_monitor/udev_util_linux.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 COMPONENTS_STORAGE_MONITOR_UDEV_UTIL_LINUX_H_
 #define COMPONENTS_STORAGE_MONITOR_UDEV_UTIL_LINUX_H_
 
 #include <string>
 
-struct udev_device;
-
 namespace base {
 class FilePath;
 }
 
 namespace storage_monitor {
 
 // Helper for udev_device_new_from_syspath()/udev_device_get_property_value()
 // pair. |device_path| is the absolute path to the device, including /sys.
 bool GetUdevDevicePropertyValueByPath(const base::FilePath& device_path,
                                       const char* key,
                                       std::string* result);
 
 }  // namespace storage_monitor
 
 #endif  // COMPONENTS_STORAGE_MONITOR_UDEV_UTIL_LINUX_H_