mash: Migrate ScopedTargetRootWindow to //ash/common

ash/scoped_target_root_window.cc

Index: ash/scoped_target_root_window.cc
diff --git a/ash/scoped_target_root_window.cc b/ash/scoped_target_root_window.cc
deleted file mode 100644
index 9213b3ad54cf56fe3f0cee0757073445ffc5604c..0000000000000000000000000000000000000000
--- a/ash/scoped_target_root_window.cc
+++ /dev/null
@@ -1,18 +0,0 @@
-// 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/scoped_target_root_window.h"
-
-#include "ash/shell.h"
-
-namespace ash {
-
-ScopedTargetRootWindow::ScopedTargetRootWindow(aura::Window* root_window) {
-  Shell::GetInstance()->scoped_target_root_window_ = root_window;
-}
-
-ScopedTargetRootWindow::~ScopedTargetRootWindow() {
-  Shell::GetInstance()->scoped_target_root_window_ = NULL;
-}
-
-}  // namespace ash