All together now

ash/display/display_controller.cc

Index: ash/display/display_controller.cc
diff --git a/ash/display/display_controller.cc b/ash/display/display_controller.cc
index f0ac711e991a5274c280290b256d5337e708051d..2442c76e7c487625c645588a98810c45f3dfb05b 100644
--- a/ash/display/display_controller.cc
+++ b/ash/display/display_controller.cc
@@ -697,6 +697,12 @@ void DisplayController::OnDisplayRemoved(const gfx::Display& display) {
   base::MessageLoop::current()->DeleteSoon(FROM_HERE, controller);
 }
 
+ui::Compositor* DisplayController::GetCompositorForDisplay(
+    const gfx::Display& display) {
+  aura::RootWindow* root_window = GetRootWindowForDisplayId(display.id());
+  return root_window ? root_window->compositor() : NULL;
+}
+
 void DisplayController::OnRootWindowHostResized(const aura::RootWindow* root) {
   internal::DisplayManager* display_manager = GetDisplayManager();
   gfx::Display display = GetDisplayNearestWindow(root);