mus: Embedder can request and observe BeginFrame for embedded client.

services/ui/ws/window_tree.cc

Index: services/ui/ws/window_tree.cc
diff --git a/services/ui/ws/window_tree.cc b/services/ui/ws/window_tree.cc
index 511a495c93d9e1159234e74f0a1f06b2825c9de6..78bf90785f0743418690749476f372ccabe783db 100644
--- a/services/ui/ws/window_tree.cc
+++ b/services/ui/ws/window_tree.cc
@@ -1573,6 +1573,20 @@ void WindowTree::SetWindowBounds(
   client()->OnChangeCompleted(change_id, success);
 }
 
+void WindowTree::RequestBeginFrames(Id window_id,
+                                    cc::mojom::FrameSinkObserverPtr observer) {
+  ServerWindow* window = GetWindowByClientId(ClientWindowId(window_id));
+  // This client must own |window_id| if it wants to be able to add a frame sink
+  // observer.
+  if (!window || window->id().client_id != id_)
+    return;
+  // This makes no sense if |window| is not embedding a client.
+  if (!window_server_->GetTreeWithRoot(window))
+    return;
+  window->GetOrCreateCompositorFrameSinkManager()->RequestBeginFrames(
+      std::move(observer));
+}
+
 void WindowTree::SetWindowVisibility(uint32_t change_id,
                                      Id transport_window_id,
                                      bool visible) {