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

ash/mus/window_manager_unittest.cc

Index: ash/mus/window_manager_unittest.cc
diff --git a/ash/mus/window_manager_unittest.cc b/ash/mus/window_manager_unittest.cc
index 5c300e9eac5d1b1f1c7d7b8fc5ea1ddce47e4e87..696671ee637ca2beaaac5409233854a86eff32e9 100644
--- a/ash/mus/window_manager_unittest.cc
+++ b/ash/mus/window_manager_unittest.cc
@@ -44,7 +44,7 @@ class WindowManagerTest : public shell::test::ShellTest {
   DISALLOW_COPY_AND_ASSIGN(WindowManagerTest);
 };
 
-void OnEmbed(bool success) {
+void OnEmbed(bool success, ::mus::mojom::InputEventHandlerPtr handler) {
   ASSERT_TRUE(success);
 }
 
@@ -131,7 +131,7 @@ TEST_F(WindowManagerTest, OpenWindow) {
   // |child_window|. This blocks until it succeeds.
   ::mus::mojom::WindowTreeClientPtr tree_client;
   auto tree_client_request = GetProxy(&tree_client);
-  child_window->Embed(std::move(tree_client), base::Bind(&OnEmbed));
+  child_window->Embed(std::move(tree_client));
   std::unique_ptr<::mus::WindowTreeClient> child_client(
       new ::mus::WindowTreeClient(&window_tree_delegate, nullptr,
                                   std::move(tree_client_request)));