[omnibox] Add a supporting comment to OnUpOrDownKeyPressed fix

CL is just adding some documentation to mahmadi's fix.

Also swaps the conditional a bit to match the comment.

Bug: 996516
Change-Id: Id69caa88bf8a243d0ffdc552c19d87080677df1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849042Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704643}

components/omnibox/browser/omnibox_edit_model.cc

@@ -1178,8 +1178,12 @@ void OmniboxEditModel::OnUpOrDownKeyPressed(int count) {
     // If, as a result of the key press, we would select the first result, then
     // we should revert the temporary text same as what pressing escape would
     // have done.
+    //
+    // This doesn't apply for on-focus suggestions, for which the first result
+    // can be completely distinct from the omnibox contents. We enforce that
+    // via user_input_in_progress_, which is false for ZeroSuggest.
     const size_t line_no = GetNewSelectedLine(count);
-    if (has_temporary_text_ && user_input_in_progress_ && line_no == 0) {
+    if (has_temporary_text_ && line_no == 0 && user_input_in_progress_) {
       RevertTemporaryText(true);
     } else {
       popup_model()->MoveTo(line_no);