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

components/mus/ws/window_tree.cc

Index: components/mus/ws/window_tree.cc
diff --git a/components/mus/ws/window_tree.cc b/components/mus/ws/window_tree.cc
index 382887111c7add1fa98b87e3c4e553d844d52632..e247a5f98a407567358b556ed2f92211633cb2ad 100644
--- a/components/mus/ws/window_tree.cc
+++ b/components/mus/ws/window_tree.cc
@@ -345,9 +345,12 @@ bool WindowTree::SetFocus(const ClientWindowId& window_id) {
 
 bool WindowTree::Embed(const ClientWindowId& window_id,
                        mojom::WindowTreeClientPtr client,
+                       mojom::InputEventHandlerRequest input_handler_request,
                        uint32_t flags) {
   if (!client || !CanEmbed(window_id))
     return false;
+  if (input_handler_request.is_pending())
+    client->RequestInputEventHandler(std::move(input_handler_request));
   ServerWindow* window = GetWindowByClientId(window_id);
   PrepareForEmbed(window);
   // When embedding we don't know the user id of where the TreeClient came
@@ -1357,8 +1360,15 @@ void WindowTree::Embed(Id transport_window_id,
                        mojom::WindowTreeClientPtr client,
                        uint32_t flags,
                        const EmbedCallback& callback) {
-  callback.Run(
-      Embed(ClientWindowId(transport_window_id), std::move(client), flags));
+  bool embedder_dispatches_event_to_embeded_client =
+      !!(flags & mojom::kEmbedFlagEmbedderInterceptsEvents);
+  mojom::InputEventHandlerPtr handler;
+  mojom::InputEventHandlerRequest request =
+      embedder_dispatches_event_to_embeded_client ? GetProxy(&handler)
+                                                  : nullptr;
+  callback.Run(Embed(ClientWindowId(transport_window_id), std::move(client),
+                     std::move(request), flags),
+               std::move(handler));
 }
 
 void WindowTree::SetFocus(uint32_t change_id, Id transport_window_id) {