mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/scoped_root_window_for_new_windows.cc

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ash/common/scoped_root_window_for_new_windows.h"
+
+#include "ash/common/wm_shell.h"
+#include "base/logging.h"
+
+namespace ash {
+
+ScopedRootWindowForNewWindows::ScopedRootWindowForNewWindows(
+    WmWindow* new_root) {
+  DCHECK(new_root);
+  WmShell::Get()->scoped_root_window_for_new_windows_ = new_root;
+}
+
+ScopedRootWindowForNewWindows::~ScopedRootWindowForNewWindows() {
+  WmShell::Get()->scoped_root_window_for_new_windows_ = nullptr;
+}
+
+}  // namespace ash