Add PlatformScreenDelegate and start implementation.

services/ui/ws/cursor_unittest.cc

Index: services/ui/ws/cursor_unittest.cc
diff --git a/services/ui/ws/cursor_unittest.cc b/services/ui/ws/cursor_unittest.cc
index 6dc720aac4cfffe8a363bd7d4b01851cd0957662..fea988b7a5e7bda06727a32ea3b1890295848aa1 100644
--- a/services/ui/ws/cursor_unittest.cc
+++ b/services/ui/ws/cursor_unittest.cc
@@ -44,9 +44,7 @@ class CursorTest : 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>(new SurfacesState)));
+    window_server_.reset(new WindowServer(&window_server_delegate_));
     window_server_delegate_.set_window_server(window_server_.get());
 
     window_server_delegate_.set_num_displays_to_create(1);