Add PlatformScreenDelegate and start implementation.

services/ui/ws/display_unittest.cc

Index: services/ui/ws/display_unittest.cc
diff --git a/services/ui/ws/display_unittest.cc b/services/ui/ws/display_unittest.cc
index 8837ebe7fcf52b988e52c5c2f6068c52f87e541d..56f8f513a0b7dec9d028406c7ae4aac0bbcb9eb7 100644
--- a/services/ui/ws/display_unittest.cc
+++ b/services/ui/ws/display_unittest.cc
@@ -62,8 +62,7 @@ class DisplayTest : 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());
     window_server_->user_id_tracker()->AddUserId(kTestId1);
     window_server_->user_id_tracker()->AddUserId(kTestId2);