Random fixes for mash bridge classes

mash/wm/bridge/mus_layout_manager_adapter.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/mus_layout_manager_adapter.h"
 
 #include "ash/wm/common/wm_layout_manager.h"
 #include "components/mus/public/cpp/window.h"
 #include "mash/wm/bridge/wm_window_mus.h"
 
@@ skipping 33 matching lines @@
 void MusLayoutManagerAdapter::OnTreeChanging(const TreeChangeParams& params) {
   if (params.old_parent == window_) {
     layout_manager_->OnWillRemoveWindowFromLayout(
         WmWindowMus::Get(params.target));
   }
 }
 
 void MusLayoutManagerAdapter::OnTreeChanged(const TreeChangeParams& params) {
   if (params.new_parent == window_) {
     layout_manager_->OnWindowAddedToLayout(WmWindowMus::Get(params.target));
+    params.target->AddObserver(&child_window_observer_);
   } else if (params.old_parent == window_) {
     params.target->RemoveObserver(&child_window_observer_);
     layout_manager_->OnWindowRemovedFromLayout(WmWindowMus::Get(params.target));
   }
 }
 
 void MusLayoutManagerAdapter::OnWindowBoundsChanged(
     mus::Window* window,
     const gfx::Rect& old_bounds,
     const gfx::Rect& new_bounds) {
   layout_manager_->OnWindowResized();
 }
 
 }  // namespace wm
 }  // namespace mash