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

mash/wm/bridge/wm_window_mus.cc

Index: mash/wm/bridge/wm_window_mus.cc
diff --git a/mash/wm/bridge/wm_window_mus.cc b/mash/wm/bridge/wm_window_mus.cc
index 97b973375c5ab3f9019684c82cc74ef9cb17825f..7818280d8098ae1c0de6c4d40429889d1655df2d 100644
--- a/mash/wm/bridge/wm_window_mus.cc
+++ b/mash/wm/bridge/wm_window_mus.cc
@@ -91,7 +91,11 @@ mus::mojom::ShowState MojomWindowShowStateFromUI(ui::WindowShowState state) {
 
 }  // 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));