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

components/mus/public/cpp/lib/window.cc

 // Copyright 2014 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 "components/mus/public/cpp/window.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <set>
@@ skipping 504 matching lines @@
 ////////////////////////////////////////////////////////////////////////////////
 // Window, private:
 
 Window::Window(WindowTreeConnection* connection, Id id)
     : connection_(connection),
       server_id_(id),
       parent_(nullptr),
       stacking_target_(nullptr),
       transient_parent_(nullptr),
       is_modal_(false),
+      // Matches aura, see aura::Window for details.
+      observers_(base::ObserverList<WindowObserver>::NOTIFY_EXISTING_ONLY),
       input_event_handler_(nullptr),
       viewport_metrics_(CreateEmptyViewportMetrics()),
       visible_(false),
       opacity_(1.0f),
       cursor_id_(mojom::Cursor::CURSOR_NULL),
       parent_drawn_(false) {}
 
 WindowTreeClientImpl* Window::tree_client() {
   return static_cast<WindowTreeClientImpl*>(connection_);
 }
@@ skipping 335 matching lines @@
     notifier->NotifyWindowReordered();
 
   return true;
 }
 
 // static
 Window** Window::GetStackingTarget(Window* window) {
   return &window->stacking_target_;
 }
 }  // namespace mus