Add FilePath to base namespace.

chrome/browser/chromeos/drive/drive_file_system_observer.h

Index: chrome/browser/chromeos/drive/drive_file_system_observer.h
diff --git a/chrome/browser/chromeos/drive/drive_file_system_observer.h b/chrome/browser/chromeos/drive/drive_file_system_observer.h
index 4bc152077ff0e88fb6094c875818f258f0fd6041..0d3a6789ee3438311e209577a13f49d903f12f63 100644
--- a/chrome/browser/chromeos/drive/drive_file_system_observer.h
+++ b/chrome/browser/chromeos/drive/drive_file_system_observer.h
@@ -7,7 +7,9 @@
 
 #include "chrome/browser/chromeos/drive/drive_file_error.h"
 
+namespace base {
 class FilePath;
+}
 
 namespace drive {
 
@@ -19,7 +21,7 @@ class DriveFileSystemObserver {
   // Triggered when a content of a directory has been changed.
   // |directory_path| is a virtual directory path (/drive/...) representing
   // changed directory.
-  virtual void OnDirectoryChanged(const FilePath& directory_path) {
+  virtual void OnDirectoryChanged(const base::FilePath& directory_path) {
   }
 
   // Triggered when the file system is initially loaded.