Add rudimentary mash shelf functionality.

mash/wm/window_manager_impl.cc

Index: mash/wm/window_manager_impl.cc
diff --git a/mash/wm/window_manager_impl.cc b/mash/wm/window_manager_impl.cc
index 5195c1f7786b0933440673b6dc632004cc40226a..84e5b2f89cb0a2084102c17f672cdaa09f149663 100644
--- a/mash/wm/window_manager_impl.cc
+++ b/mash/wm/window_manager_impl.cc
@@ -123,6 +123,15 @@ void WindowManagerImpl::OnTreeChanging(const TreeChangeParams& params) {
     params.target->RemoveObserver(this);
   else if (state_->WindowIsContainer(params.new_parent))
     params.target->AddObserver(this);
+
+  if (user_window_observer_) {
+    mus::Window* user_window_container =
+        state_->GetWindowForContainer(mojom::CONTAINER_USER_WINDOWS);
+    if (params.new_parent == user_window_container)
+      user_window_observer_->OnUserWindowAdded(params.target->id());
+    else if (params.old_parent == user_window_container)
+      user_window_observer_->OnUserWindowRemoved(params.target->id());
+  }
 }
 
 void WindowManagerImpl::OnWindowEmbeddedAppDisconnected(mus::Window* window) {
@@ -167,6 +176,20 @@ void WindowManagerImpl::GetConfig(const GetConfigCallback& callback) {
   callback.Run(std::move(config));
 }
 
+void WindowManagerImpl::AddUserWindowObserver(
+    mus::mojom::UserWindowObserverPtr observer) {
+  // TODO(msw): Support multiple observers.
+  user_window_observer_ = std::move(observer);
+}
+
+void WindowManagerImpl::FocusUserWindow(uint32_t window_id) {
+  mus::Window* container =
+      state_->GetWindowForContainer(mojom::CONTAINER_USER_WINDOWS);
+  mus::Window* window = container->GetChildById(window_id);
+  if (window)
+    window->SetFocus();
+}
+
 bool WindowManagerImpl::OnWmSetBounds(mus::Window* window, gfx::Rect* bounds) {
   // By returning true the bounds of |window| is updated.
   return true;