Use persistent ID/names for displays.

ui/aura/single_display_manager.cc

Index: ui/aura/single_display_manager.cc
diff --git a/ui/aura/single_display_manager.cc b/ui/aura/single_display_manager.cc
index 384c3c32c10f9ac654a23d3bf45b669d3139849f..307868658036b198ad5a1eb8b370bb777a2787f4 100644
--- a/ui/aura/single_display_manager.cc
+++ b/ui/aura/single_display_manager.cc
@@ -77,6 +77,10 @@ const gfx::Display& SingleDisplayManager::GetDisplayMatching(
   return display_;
 }
 
+std::string SingleDisplayManager::GetDisplayNameAt(size_t index) {
+  return "Display";
+}
+
 void SingleDisplayManager::OnWindowBoundsChanged(
     Window* window, const gfx::Rect& old_bounds, const gfx::Rect& new_bounds) {
   if (!use_fullscreen_host_window()) {