Update comment in FileSystemFileUtil

Bug: 948446
Change-Id: Iea4d98b54b20960126f96d17ba8cfa34562f902c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707134
Auto-Submit: Sergei Datsenko <dats@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678575}

storage/browser/fileapi/file_system_file_util.h

@@ -98,7 +98,8 @@ class COMPONENT_EXPORT(STORAGE_BROWSER) FileSystemFileUtil {
   // current call returns to the main FILE message loop.
   //
   // The supplied context must remain valid at least lifetime of the enumerator
-  // instance.
+  // instance. |this| must remain valid through the lifetime of the created
+  // enumerator instance.
   virtual std::unique_ptr<AbstractFileEnumerator> CreateFileEnumerator(
       FileSystemOperationContext* context,
       const FileSystemURL& root_url,