Add FilePath to base namespace.

chrome/browser/chromeos/drive/file_system/operation_observer.h

 // Copyright (c) 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 CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_OPERATION_OBSERVER_H_
 #define CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_OPERATION_OBSERVER_H_
 
+namespace base {
 class FilePath;
+}
 
 namespace drive {
 namespace file_system {
 
 // Passes notifications from Drive operations back to the file system.
 class OperationObserver {
  public:
   // Sent when a content of a directory has been changed.
   // |directory_path| is a virtual directory path representing the
   // changed directory.
   virtual void OnDirectoryChangedByOperation(
-      const FilePath& directory_path) = 0;
+      const base::FilePath& directory_path) = 0;
 };
 
 }  // namespace file_system
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_OPERATION_OBSERVER_H_