Fix suspicious code detected by closure compiler.

chrome/common/extensions/api/file_manager_private.idl

Index: chrome/common/extensions/api/file_manager_private.idl
diff --git a/chrome/common/extensions/api/file_manager_private.idl b/chrome/common/extensions/api/file_manager_private.idl
index 6223cffc97ec881b51319c5ddb75331dd206fccd..cf14c43b7b6aaa2c73440c078a11116d95058c31 100644
--- a/chrome/common/extensions/api/file_manager_private.idl
+++ b/chrome/common/extensions/api/file_manager_private.idl
@@ -665,10 +665,11 @@ interface Functions {
 
   // Cancels ongoing file transfers for selected files.
   // |fileUrls| Array of files for which ongoing transfer should be canceled.
-  //     If this is absent, all jobs are canceled. 
-  // |callback|
-  static void cancelFileTransfers(optional DOMString[] fileUrls,
-                                  CancelFileTransfersCallback callback);
+  //     If this is absent, all jobs are canceled.
+  // |callback| Completion callback of the cancel.
+  static void cancelFileTransfers(
+      optional DOMString[] fileUrls,
+      optional CancelFileTransfersCallback callback);
 
   // Starts to copy an entry. If the source is a directory, the copy is done
   // recursively.