Random fixes for mash bridge classes

mash/wm/bridge/wm_lookup_mus.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 "mash/wm/bridge/wm_lookup_mus.h"
 
 #include "mash/wm/bridge/wm_globals_mus.h"
 #include "mash/wm/bridge/wm_root_window_controller_mus.h"
 #include "mash/wm/bridge/wm_window_mus.h"
 #include "ui/views/widget/widget.h"
 
 namespace mash {
 namespace wm {
 
-WmLookupMus::WmLookupMus() {}
+WmLookupMus::WmLookupMus() {
+  WmLookup::Set(this);
+}
 
-WmLookupMus::~WmLookupMus() {}
+WmLookupMus::~WmLookupMus() {
+  if (WmLookupMus::Get() == this)
+    WmLookup::Set(nullptr);
+}
 
 ash::wm::WmRootWindowController*
 WmLookupMus::GetRootWindowControllerWithDisplayId(int64_t id) {
   return WmGlobalsMus::Get()->GetRootWindowControllerWithDisplayId(id);
 }
 
 ash::wm::WmWindow* WmLookupMus::GetWindowForWidget(views::Widget* widget) {
   return WmWindowMus::Get(widget);
 }
 
 }  // namespace wm
 }  // namespace mash