Add PlatformScreenDelegate and start implementation.

services/ui/ws/user_display_manager_unittest.cc

Index: services/ui/ws/user_display_manager_unittest.cc
diff --git a/services/ui/ws/user_display_manager_unittest.cc b/services/ui/ws/user_display_manager_unittest.cc
index 8fed30133c4c9821432b03aec54bee377b71c486..db5badde0ed45f09f0676f906180bacadab50bee 100644
--- a/services/ui/ws/user_display_manager_unittest.cc
+++ b/services/ui/ws/user_display_manager_unittest.cc
@@ -99,8 +99,7 @@ class UserDisplayManagerTest : public testing::Test {
   // testing::Test:
   void SetUp() override {
     PlatformDisplay::set_factory_for_testing(&platform_display_factory_);
-    window_server_.reset(new WindowServer(&window_server_delegate_,
-                                          scoped_refptr<SurfacesState>()));
+    window_server_.reset(new WindowServer(&window_server_delegate_));
     window_server_delegate_.set_window_server(window_server_.get());
   }