Moves MruWindowTracker to WmShell

ash/common/shell_common.cc

+// Copyright 2016 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/shell_common.h"
+
+#include "ash/common/wm/mru_window_tracker.h"
+
+namespace ash {
+
+ShellCommon::ShellCommon() {}
+
+ShellCommon::~ShellCommon() {}
+
+void ShellCommon::CreateMruWindowTracker() {
+  mru_window_tracker_.reset(new MruWindowTracker);
+}
+
+void ShellCommon::DeleteMruWindowTracker() {
+  mru_window_tracker_.reset();
+}
+
+}  // namespace ash