Use persistent ID/names for displays.

ash/display/display_controller.h

Index: ash/display/display_controller.h
diff --git a/ash/display/display_controller.h b/ash/display/display_controller.h
index 45fa80136a4313f8034ac079c00566f882cc5738..23ed86c45a7c8e3e6aab567def45f3f4f5020503 100644
--- a/ash/display/display_controller.h
+++ b/ash/display/display_controller.h
@@ -100,11 +100,15 @@ class ASH_EXPORT DisplayController : public aura::DisplayObserver {
 
   // Creates a root window for |display| and stores it in the |root_windows_|
   // map.
-  aura::RootWindow* AddRootWindowForDisplay(const gfx::Display& display);
+  // TODO(oshima): remove |is_primary| when non extended desktop mode is
+  // removed.
+  aura::RootWindow* AddRootWindowForDisplay(const gfx::Display& display,
+                                            bool is_primary);
 
   void UpdateDisplayBoundsForLayout();
 
-  std::map<int, aura::RootWindow*> root_windows_;
+  // The mapping from display ID to its root window.
+  std::map<int64, aura::RootWindow*> root_windows_;
 
   SecondaryDisplayLayout secondary_display_layout_;