mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/accelerators/exit_warning_handler.cc

Index: ash/common/accelerators/exit_warning_handler.cc
diff --git a/ash/common/accelerators/exit_warning_handler.cc b/ash/common/accelerators/exit_warning_handler.cc
index d0c0a6187f9e010ed33b651faafb7fa2112a11d9..6b54f768fd87eeffb6a5fa50f6e7dd21e2e49b43 100644
--- a/ash/common/accelerators/exit_warning_handler.cc
+++ b/ash/common/accelerators/exit_warning_handler.cc
@@ -146,7 +146,7 @@ void ExitWarningHandler::CancelTimer() {
 void ExitWarningHandler::Show() {
   if (widget_)
     return;
-  WmWindow* root_window = WmShell::Get()->GetRootWindowForNewWindows();
+  WmWindow* root_window = WmShell::Get()->root_window_for_new_windows();
   ExitWarningWidgetDelegateView* delegate = new ExitWarningWidgetDelegateView;
   gfx::Size rs = root_window->GetBounds().size();
   gfx::Size ps = delegate->GetPreferredSize();