cros: Expose Assign to Desk menu for PWAs in context menu.

With browsers, users can access the Assign to Desk menu through the
context menu. For PWAs, users can only access the Assign to Desk menu
through the triple dot menu. Users should be able to access this menu
through the context menu for PWAs as well.

This CL exposes the Assign to Desk menu in PWA's context menu.

Test: manual
Bug: 1164003e
Change-Id: I1d0b4c2463aa1e4d56e75ecc7122d231c4acd25f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616343Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jeremy Chinsen <chinsenj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841293}

chrome/browser/ui/views/frame/system_menu_model_builder.cc

@@ -133,6 +133,9 @@ void SystemMenuModelBuilder::BuildSystemMenuForAppOrPopupWindow(
 #if defined(OS_LINUX) || BUILDFLAG(IS_CHROMEOS_LACROS)
   model->AddSeparator(ui::NORMAL_SEPARATOR);
   model->AddItemWithStringId(IDC_CLOSE_WINDOW, IDS_CLOSE);
+#endif
+#if BUILDFLAG(IS_CHROMEOS_ASH)
+  AppendAssignToDesksMenu(model);
 #endif
   AppendTeleportMenu(model);
 }