Moves MruWindowTracker to WmShell

ash/common/shell_common.cc

Index: ash/common/shell_common.cc
diff --git a/ash/common/shell_common.cc b/ash/common/shell_common.cc
new file mode 100644
index 0000000000000000000000000000000000000000..16507391ecf7b7e53e3d77d48500b4797cdaa9fe
--- /dev/null
+++ b/ash/common/shell_common.cc
@@ -0,0 +1,23 @@
+// 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