ash: Add RootWindowHostFactory class.

ash/display/display_manager.cc

Index: ash/display/display_manager.cc
diff --git a/ash/display/display_manager.cc b/ash/display/display_manager.cc
index e14d39a890aee83acc2731cda1ecea0f82528ed3..d057b473f3f3f05b537221a9731f61fe6917c46b 100644
--- a/ash/display/display_manager.cc
+++ b/ash/display/display_manager.cc
@@ -81,9 +81,10 @@ using std::vector;
 DEFINE_WINDOW_PROPERTY_KEY(int64, kDisplayIdKey,
                            gfx::Display::kInvalidDisplayID);
 
-DisplayManager::DisplayManager() :
+DisplayManager::DisplayManager(DisplayManagerDelegate* delegate) :
     internal_display_id_(gfx::Display::kInvalidDisplayID),
-    force_bounds_changed_(false) {
+    force_bounds_changed_(false),
+    delegate_(delegate) {
   Init();
 }
 
@@ -320,12 +321,8 @@ void DisplayManager::UpdateDisplays(
 RootWindow* DisplayManager::CreateRootWindowForDisplay(
     const gfx::Display& display) {
   RootWindow::CreateParams params(display.bounds_in_pixel());
-#if defined(OS_WIN)
-  if (base::win::GetVersion() >= base::win::VERSION_WIN8) {
-    params.host = aura::RemoteRootWindowHostWin::Create(
-        display.bounds_in_pixel());
-  }
-#endif
+  DCHECK(delegate_);
+  params.delegate = delegate_->CreateRootWindowDelegate();
   aura::RootWindow* root_window = new aura::RootWindow(params);
   // No need to remove RootWindowObserver because
   // the DisplayManager object outlives RootWindow objects.