drive: Fix a crash caused by an empty edit URL

chrome/browser/chromeos/gdata/drive_file_system_interface.h

Index: chrome/browser/chromeos/gdata/drive_file_system_interface.h
diff --git a/chrome/browser/chromeos/gdata/drive_file_system_interface.h b/chrome/browser/chromeos/gdata/drive_file_system_interface.h
index 0749a8ddfc3fd9a1191be8038f374ffd4868075b..562d01e12acc63d8635a0d1c63e82e577c3e15bb 100644
--- a/chrome/browser/chromeos/gdata/drive_file_system_interface.h
+++ b/chrome/browser/chromeos/gdata/drive_file_system_interface.h
@@ -227,6 +227,7 @@ class DriveFileSystemInterface {
   // TODO(satorux): is_recursive is not supported yet. crbug.com/138282
   //
   // Can be called from UI/IO thread. |callback| is run on the calling thread.
+  // |callback| must not be null.
   virtual void Remove(const FilePath& file_path,
                       bool is_recursive,
                       const FileOperationCallback& callback) = 0;