Moves move logic into WM instead of WS

components/mus/example/wm/window_manager_application.cc

Index: components/mus/example/wm/window_manager_application.cc
diff --git a/components/mus/example/wm/window_manager_application.cc b/components/mus/example/wm/window_manager_application.cc
index 2ab82f8fa8bafe2225ee016e228a3d9b463c7ba5..175397e79bc78d766cf9a77c87acb79cb7196a01 100644
--- a/components/mus/example/wm/window_manager_application.cc
+++ b/components/mus/example/wm/window_manager_application.cc
@@ -43,13 +43,11 @@ bool WindowManagerApplication::ConfigureIncomingConnection(
 
 void WindowManagerApplication::OnEmbed(mus::Window* root) {
   root_ = root;
+  root_->AddObserver(this);
   CreateContainers();
   layout_.reset(
       new WindowLayout(GetWindowForContainer(Container::USER_WINDOWS)));
 
-  host_->EnableWindowDraggingForChildren(
-      GetWindowForContainer(Container::USER_WINDOWS)->id());
-
   window_manager_.reset(new WindowManagerImpl(this));
   for (auto request : requests_)
     window_manager_binding_.AddBinding(window_manager_.get(), request->Pass());
@@ -73,6 +71,15 @@ void WindowManagerApplication::Create(
   }
 }
 
+void WindowManagerApplication::OnWindowDestroyed(mus::Window* window) {
+  DCHECK_EQ(window, root_);
+  root_->RemoveObserver(this);
+  // Delete the |window_manager_| here so that WindowManager doesn't have to
+  // worry about the possibility of |root_| being null.
+  window_manager_.reset();
+  root_ = nullptr;
+}
+
 void WindowManagerApplication::CreateContainers() {
   for (uint16_t container =
            static_cast<uint16_t>(Container::ALL_USER_BACKGROUND);