mus: Introduce API for embedder to dispatch event to the embeded client.

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

Index: components/mus/public/cpp/lib/window.cc
diff --git a/components/mus/public/cpp/lib/window.cc b/components/mus/public/cpp/lib/window.cc
index 83785c4544e148ee9d2f05e231b38c4c7e35a2c8..86ca0dee8b43990382d95a11ddbd1c948e18e357 100644
--- a/components/mus/public/cpp/lib/window.cc
+++ b/components/mus/public/cpp/lib/window.cc
@@ -161,7 +161,7 @@ bool OwnsWindowOrIsRoot(Window* window) {
   return OwnsWindow(window->window_tree(), window) || IsClientRoot(window);
 }
 
-void EmptyEmbedCallback(bool result) {}
+void EmptyEmbedCallback(bool result, mojom::InputEventHandlerPtr handler) {}
 
 }  // namespace
 
@@ -457,7 +457,7 @@ void Window::Embed(mus::mojom::WindowTreeClientPtr client,
   if (PrepareForEmbed())
     client_->Embed(server_id_, std::move(client), flags, callback);
   else
-    callback.Run(false);
+    callback.Run(false, nullptr);
 }
 
 void Window::RequestClose() {