Remove DisplayChangeObserver ash dependencies.

ash/display/screen_ash.cc

Index: ash/display/screen_ash.cc
diff --git a/ash/display/screen_ash.cc b/ash/display/screen_ash.cc
index f37d756ef1d96d856b371ecafaf74f7732d4c5da..aabf4cf3ce82c0f4adee308ae9cf7caa5830d11a 100644
--- a/ash/display/screen_ash.cc
+++ b/ash/display/screen_ash.cc
@@ -7,6 +7,7 @@
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/shelf/shelf_widget.h"
 #include "ash/common/wm/root_window_finder.h"
+#include "ash/display/ash_display_string_provider.h"
 #include "ash/display/window_tree_host_manager.h"
 #include "ash/root_window_controller.h"
 #include "ash/root_window_settings.h"
@@ -185,7 +186,8 @@ display::DisplayManager* ScreenAsh::CreateDisplayManager() {
   // use ash's screen.
   if (!current || current == screen_for_shutdown)
     display::Screen::SetScreenInstance(screen.get());
-  return new display::DisplayManager(std::move(screen));
+  return new display::DisplayManager(
+      std::move(screen), base::MakeUnique<AshDisplayStringProvider>());
 }
 
 // static