Fix the visibility of embedded assistant UI in tablet mode.

As commented in the code, page_view->RequestFocus() might cause
ResetForShow(), which hides the embedded asssitant view, thus
the assistant UI might go invisible in a weird situation.

I hope I could find a holistic solution of controlling states
within ContentsView, but so far this would be the simplest fix
for the problem.

Bug: b/140831868
Test: manually
Change-Id: I9e1b551c2db448ff40c9f978130ce2a0225a4ebb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803958
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696597}

ash/app_list/views/contents_view.cc

@@ -317,8 +317,14 @@ void ContentsView::ShowEmbeddedAssistantUI(bool show) {
   // Hide or Show results.
   auto* page_view = GetPageView(assistant_page);
   page_view->SetVisible(show);
-  if (show)
+  if (show) {
     page_view->RequestFocus();
+    // RequestFocus() might cause ResetForShow() method through
+    // AppListView::OnHomeLauncherGainingFocusWithoutAnimation() and it can hide
+    // |page_view|. Thus |page_view|'s visibility should be set again. See
+    // b/140831868.
+    page_view->SetVisible(show);
+  }
 
   const int search_results_page =
       GetPageIndexForState(ash::AppListState::kStateSearchResults);