Nix GetRootWindowController, use RootWindowController::ForWindow.

ash/wm_window.cc

Index: ash/wm_window.cc
diff --git a/ash/wm_window.cc b/ash/wm_window.cc
index aea1e78b6a8e78458ab88749ef2bdc2fcc07881d..619975b8350938914f1902efe7fd049859518ae6 100644
--- a/ash/wm_window.cc
+++ b/ash/wm_window.cc
@@ -8,7 +8,6 @@
 #include "ash/public/cpp/config.h"
 #include "ash/public/cpp/shell_window_ids.h"
 #include "ash/public/cpp/window_properties.h"
-#include "ash/root_window_controller.h"
 #include "ash/shell.h"
 #include "ash/wm/resize_handle_window_targeter.h"
 #include "ash/wm/widget_finder.h"
@@ -106,11 +105,6 @@ const WmWindow* WmWindow::GetRootWindow() const {
   return Get(window_->GetRootWindow());
 }
 
-RootWindowController* WmWindow::GetRootWindowController() {
-  aura::Window* root = window_->GetRootWindow();
-  return root ? RootWindowController::ForWindow(root) : nullptr;
-}
-
 aura::client::WindowType WmWindow::GetType() const {
   return window_->type();
 }