Makes mus::Window and mash::wm::WmWindowMus notify existing observers only

mash/wm/bridge/wm_window_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_window_mus.h"
 
 #include "ash/wm/common/container_finder.h"
 #include "ash/wm/common/window_state.h"
 #include "ash/wm/common/wm_layout_manager.h"
 #include "ash/wm/common/wm_window_observer.h"
@@ skipping 73 matching lines @@
     case ui::SHOW_STATE_DOCKED:
       return mus::mojom::ShowState::DOCKED;
     default:
       break;
   }
   return mus::mojom::ShowState::DEFAULT;
 }
 
 }  // namespace
 
-WmWindowMus::WmWindowMus(mus::Window* window) : window_(window) {
+WmWindowMus::WmWindowMus(mus::Window* window)
+    : window_(window),
+      // Matches aura, see aura::Window for details.
+      observers_(
+          base::ObserverList<ash::wm::WmWindowObserver>::NOTIFY_EXISTING_ONLY) {
   window_->AddObserver(this);
   window_->SetLocalProperty(kWmWindowKey, this);
   window_state_.reset(new WindowStateMus(this));
 }
 
 WmWindowMus::~WmWindowMus() {
   window_->RemoveObserver(this);
 }
 
 // static
@@ skipping 488 matching lines @@
                     OnWindowBoundsChanged(this, old_bounds, new_bounds));
 }
 
 void WmWindowMus::OnWindowDestroying(mus::Window* window) {
   FOR_EACH_OBSERVER(ash::wm::WmWindowObserver, observers_,
                     OnWindowDestroying(this));
 }
 
 }  // namespace wm
 }  // namespace mash