mus: Fix ash::GetRootWindowController for child windows/widgets

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index d4e9d54f46839f0cdd8618e43dbd7a68b361a19d..7d6b190fae017942f508c57aa6184bc199f63af3 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -1056,7 +1056,22 @@ void RootWindowController::OnTouchHudProjectionToggled(bool enabled) {
 
 RootWindowController* GetRootWindowController(
     const aura::Window* root_window) {
-  return root_window ? GetRootWindowSettings(root_window)->controller : NULL;
+  if (!root_window)
+    return nullptr;
+
+  if (Shell::GetInstance()->in_mus()) {
+    // On mus, like desktop aura, each top-level widget has its own root window,
+    // so |root_window| is not necessarily the display's root. For now, just
+    // the use the primary display root.
+    // TODO(jamescook): Multi-display support. This depends on how mus windows
+    // will be owned by displays.
+    aura::Window* primary_root_window = Shell::GetInstance()
+                                            ->window_tree_host_manager()
+                                            ->GetPrimaryRootWindow();
+    return GetRootWindowSettings(primary_root_window)->controller;
+  }
+
+  return GetRootWindowSettings(root_window)->controller;
 }
 
 }  // namespace ash