Moves maintaining root_window_for_new_windows_ to Shell

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 f621e8057ae19b9a32e3ed749acc0549d112234c..1197d1e04623b517f9c2ed09a9d8458fc9b8b62e 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -1124,7 +1124,7 @@ void RootWindowController::DisableTouchHudProjection() {
 }
 
 void RootWindowController::ResetRootForNewWindowsIfNecessary() {
-  WmShell* shell = WmShell::Get();
+  Shell* shell = Shell::GetInstance();
   // Change the target root window before closing child windows. If any child
   // being removed triggers a relayout of the shelf it will try to build a
   // window list adding windows from the target root window's containers which
@@ -1133,7 +1133,7 @@ void RootWindowController::ResetRootForNewWindowsIfNecessary() {
   if (shell->GetRootWindowForNewWindows() == root) {
     // The root window for new windows is being destroyed. Switch to the primary
     // root window if possible.
-    WmWindow* primary_root = shell->GetPrimaryRootWindow();
+    WmWindow* primary_root = WmShell::Get()->GetPrimaryRootWindow();
     shell->set_root_window_for_new_windows(primary_root == root ? nullptr
                                                                 : primary_root);
   }