[Autofill Assistant] Reserve field in ShowGenericUi proto

Change-Id: Id59efac059eea817650c5c9c3163ac372d1df875
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352919Reviewed-by: Clemens Arbesser <arbesser@google.com>
Commit-Queue: Luca Hunkeler <hluca@google.com>
Cr-Commit-Position: refs/heads/master@{#798065}

components/autofill_assistant/browser/service.proto

@@ -1393,6 +1393,8 @@ message ShowGenericUiProto {
   // requested values is missing. Note that all values will have
   // |is_client_side_only| set to true.
   optional RequestUserData request_user_data = 9;
+
+  reserved 10;
 }
 
 // Allow choosing one or more possibility. If FocusElement was called just