Remove ui/gfx/screen_type_delegate.h

This removes gfx::SCREEN_TYPE_NATIVE/SCREEN_TYPE_ALTERNATE, and there is
now only one Screen instance. It's still a runtime-installed thing for
test purposes.

TBR=various owners for trivial changes
[email protected], [email protected]
BUG=558054

Review URL: https://codereview.chromium.org/1608733002

Cr-Commit-Position: refs/heads/master@{#371676}
diff --git a/ui/aura/demo/demo_main.cc b/ui/aura/demo/demo_main.cc
index f485a8c..4a2467f 100644
--- a/ui/aura/demo/demo_main.cc
+++ b/ui/aura/demo/demo_main.cc
@@ -150,7 +150,7 @@
   aura::Env::GetInstance()->set_context_factory(context_factory.get());
   scoped_ptr<aura::TestScreen> test_screen(
       aura::TestScreen::Create(gfx::Size()));
-  gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_NATIVE, test_screen.get());
+  gfx::Screen::SetScreenInstance(test_screen.get());
   scoped_ptr<aura::WindowTreeHost> host(
       test_screen->CreateHostForPrimaryDisplay());
   scoped_ptr<DemoWindowTreeClient> window_tree_client(