assistant: only show assistant button when native

Bug: 879401
Test: eve does not have assistant button
Change-Id: Ie65d700eb6c052d2089bef6ec2a209bdd12dbc9b
Reviewed-on: https://chromium-review.googlesource.com/1198282Reviewed-by: Weidong Guo <weidongg@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587876}

ash/app_list/app_list_controller_impl.cc

@@ -804,6 +804,9 @@ void AppListControllerImpl::UpdateHomeLauncherVisibility() {
 }
 
 void AppListControllerImpl::UpdateAssistantVisibility() {
+  if (!chromeos::switches::IsAssistantEnabled())
+    return;
+
   auto* controller = Shell::Get()->voice_interaction_controller();
   GetSearchModel()->search_box()->SetShowAssistantButton(
       controller->settings_enabled() &&