mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/scoped_root_window_for_new_windows.cc

Index: ash/common/scoped_root_window_for_new_windows.cc
diff --git a/ash/common/scoped_root_window_for_new_windows.cc b/ash/common/scoped_root_window_for_new_windows.cc
new file mode 100644
index 0000000000000000000000000000000000000000..28e24f00f6a7c0b9efe26e2b924d2e1278a5802e
--- /dev/null
+++ b/ash/common/scoped_root_window_for_new_windows.cc
@@ -0,0 +1,22 @@
+// 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