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

ash/wm/panels/panel_layout_manager.cc

Index: ash/wm/panels/panel_layout_manager.cc
diff --git a/ash/wm/panels/panel_layout_manager.cc b/ash/wm/panels/panel_layout_manager.cc
index 56d8e5a91233ea9e62e51410723ee3c4b49c92d9..fc2f4bb90b3a51c94fa64228aaefac16ab09fae8 100644
--- a/ash/wm/panels/panel_layout_manager.cc
+++ b/ash/wm/panels/panel_layout_manager.cc
@@ -450,8 +450,7 @@ void PanelLayoutManager::OnLauncherIconPositionsChanged() {
 ////////////////////////////////////////////////////////////////////////////////
 // PanelLayoutManager, ash::ShellObserver implementation:
 
-void PanelLayoutManager::OnShelfAlignmentChanged(
-    aura::RootWindow* root_window) {
+void PanelLayoutManager::OnShelfAlignmentChanged(aura::Window* root_window) {
   if (panel_container_->GetRootWindow() == root_window)
     Relayout();
 }