Make GetRootWindow() return a Window instead of a RootWindow.

ash/wm/dock/docked_window_layout_manager.cc

Index: ash/wm/dock/docked_window_layout_manager.cc
diff --git a/ash/wm/dock/docked_window_layout_manager.cc b/ash/wm/dock/docked_window_layout_manager.cc
index 3307d6c95209fc98fc416f0b561322bba4884a97..784666694900ced0de155e9ca905903772912053 100644
--- a/ash/wm/dock/docked_window_layout_manager.cc
+++ b/ash/wm/dock/docked_window_layout_manager.cc
@@ -501,7 +501,7 @@ void DockedWindowLayoutManager::OnDisplayWorkAreaInsetsChanged() {
 }
 
 void DockedWindowLayoutManager::OnFullscreenStateChanged(
-    bool is_fullscreen, aura::RootWindow* root_window) {
+    bool is_fullscreen, aura::Window* root_window) {
   if (dock_container_->GetRootWindow() != root_window)
     return;
   // Entering fullscreen mode (including immersive) hides docked windows.
@@ -533,7 +533,7 @@ void DockedWindowLayoutManager::OnFullscreenStateChanged(
 }
 
 void DockedWindowLayoutManager::OnShelfAlignmentChanged(
-    aura::RootWindow* root_window) {
+    aura::Window* root_window) {
   if (dock_container_->GetRootWindow() != root_window)
     return;