Removes some dead code

ash/host/ash_window_tree_host.cc

Index: ash/host/ash_window_tree_host.cc
diff --git a/ash/host/ash_window_tree_host.cc b/ash/host/ash_window_tree_host.cc
index 73ecd8cc6ead963f989a76bc4713973afedc73bf..939f097d35648af36c2c33d1b4c972f8e1cb175b 100644
--- a/ash/host/ash_window_tree_host.cc
+++ b/ash/host/ash_window_tree_host.cc
@@ -6,7 +6,6 @@
 
 #include "ash/host/ash_window_tree_host_init_params.h"
 #include "ash/host/ash_window_tree_host_unified.h"
-#include "base/lazy_instance.h"
 #include "ui/aura/client/screen_position_client.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/events/event.h"
@@ -16,19 +15,10 @@
 #include "ash/host/ash_window_tree_host_platform.h"
 #elif defined(USE_X11)
 #include "ash/host/ash_window_tree_host_x11.h"
-#elif defined(OS_WIN)
-#include "ash/host/ash_window_tree_host_win.h"
 #endif
 
 namespace ash {
 
-namespace {
-
-base::LazyInstance<AshWindowTreeHost::Factory> creation_factory =
-    LAZY_INSTANCE_INITIALIZER;
-
-}  // namespace
-
 AshWindowTreeHost::AshWindowTreeHost() : input_method_handler_(nullptr) {}
 
 void AshWindowTreeHost::TranslateLocatedEvent(ui::LocatedEvent* event) {
@@ -59,25 +49,15 @@ void AshWindowTreeHost::TranslateLocatedEvent(ui::LocatedEvent* event) {
 // static
 AshWindowTreeHost* AshWindowTreeHost::Create(
     const AshWindowTreeHostInitParams& init_params) {
-  if (!creation_factory.Get().is_null())
-    return creation_factory.Get().Run(init_params);
-
   if (init_params.offscreen)
     return new AshWindowTreeHostUnified(init_params.initial_bounds);
 #if defined(USE_OZONE)
   return new AshWindowTreeHostPlatform(init_params.initial_bounds);
 #elif defined(USE_X11)
   return new AshWindowTreeHostX11(init_params.initial_bounds);
-#elif defined(OS_WIN)
-  return new AshWindowTreeHostWin(init_params.initial_bounds);
 #else
 #error Unsupported platform.
 #endif
 }
 
-// static
-void AshWindowTreeHost::SetFactory(const Factory& factory) {
-  creation_factory.Get() = factory;
-}
-
 }  // namespace ash