mash: remove more shell/shelf WmWindow usage.

ash/system/toast/toast_overlay.cc

Index: ash/system/toast/toast_overlay.cc
diff --git a/ash/system/toast/toast_overlay.cc b/ash/system/toast/toast_overlay.cc
index d87952106741a92478f631b6d1ed54d005321aeb..80eb27f2bf6f096a9a2aa07d3af5dfd7fe717048 100644
--- a/ash/system/toast/toast_overlay.cc
+++ b/ash/system/toast/toast_overlay.cc
@@ -50,7 +50,7 @@ const int kToastMinimumWidth = 288;
 // Returns the work area bounds for the root window where new windows are added
 // (including new toasts).
 gfx::Rect GetUserWorkAreaBounds() {
-  return WmShelf::ForWindow(Shell::GetWmRootWindowForNewWindows())
+  return WmShelf::ForWindow(Shell::GetRootWindowForNewWindows())
       ->GetUserWorkAreaBounds();
 }
 
@@ -216,8 +216,7 @@ ToastOverlay::ToastOverlay(Delegate* delegate,
   params.remove_standard_frame = true;
   params.bounds = CalculateOverlayBounds();
   // Show toasts above the app list and below the lock screen.
-  Shell::GetWmRootWindowForNewWindows()
-      ->GetRootWindowController()
+  GetRootWindowController(Shell::GetRootWindowForNewWindows())
       ->ConfigureWidgetInitParamsForContainer(
           overlay_widget_.get(), kShellWindowId_SystemModalContainer, &params);
   overlay_widget_->Init(params);