Reset the experimental app list to the start page on dismiss.

This CL fixes and issue where the experimental app list would not
reset to the start page on Windows and Linux.

BUG=412227

Review URL: https://codereview.chromium.org/552743005

Cr-Commit-Position: refs/heads/master@{#294307}

ui/app_list/views/app_list_main_view.cc

@@ -186,6 +186,10 @@ void AppListMainView::ShowAppListWhenReady() {
 }
 
 void AppListMainView::ResetForShow() {
+  if (switches::IsExperimentalAppListEnabled()) {
+    contents_view_->SetActivePage(contents_view_->GetPageIndexForNamedPage(
+        ContentsView::NAMED_PAGE_START));
+  }
   contents_view_->apps_container_view()->ResetForShowApps();
   // We clear the search when hiding so when app list appears it is not showing
   // search results.