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

components/mus/public/cpp/window_tree_client.h

Index: components/mus/public/cpp/window_tree_client.h
diff --git a/components/mus/public/cpp/window_tree_client.h b/components/mus/public/cpp/window_tree_client.h
index 24ef564f3f580368956a21ac3c6bca19b0b36671..ab9073a07238e8b47be4921274d52d1154904ea4 100644
--- a/components/mus/public/cpp/window_tree_client.h
+++ b/components/mus/public/cpp/window_tree_client.h
@@ -59,6 +59,7 @@ enum class ChangeType;
 // WindowTreeClientDelegate::OnWindowTreeClientDestroyed().
 class WindowTreeClient : public mojom::WindowTreeClient,
                          public mojom::WindowManager,
+                         public mojom::InputEventHandler,
                          public WindowManagerClient {
  public:
   WindowTreeClient(WindowTreeClientDelegate* delegate,
@@ -300,6 +301,8 @@ class WindowTreeClient : public mojom::WindowTreeClient,
   void OnWindowPredefinedCursorChanged(Id window_id,
                                        mojom::Cursor cursor) override;
   void OnChangeCompleted(uint32_t change_id, bool success) override;
+  void RequestInputEventHandler(
+      mojom::InputEventHandlerRequest request) override;
   void RequestClose(uint32_t window_id) override;
   void GetWindowManager(
       mojo::AssociatedInterfaceRequest<WindowManager> internal) override;
@@ -341,6 +344,11 @@ class WindowTreeClient : public mojom::WindowTreeClient,
       const gfx::Vector2d& offset,
       const gfx::Insets& hit_area) override;
 
+  // mojom::InputEventHandler:
+  void OnInputEvent(uint32_t window_id,
+                    std::unique_ptr<ui::Event> event,
+                    const OnInputEventCallback& callback) override;
+
   // The one int in |cursor_location_mapping_|. When we read from this
   // location, we must always read from it atomically.
   base::subtle::Atomic32* cursor_location_memory() {
@@ -391,6 +399,7 @@ class WindowTreeClient : public mojom::WindowTreeClient,
   std::unique_ptr<mojo::AssociatedBinding<mojom::WindowManager>>
       window_manager_internal_;
   mojom::WindowManagerClientAssociatedPtr window_manager_internal_client_;
+  mojo::Binding<mus::mojom::InputEventHandler> event_handler_binding_;
 
   bool has_event_observer_ = false;