Move root_window_host* in aura to window_tree_host*.

ash/display/display_controller.cc

Index: ash/display/display_controller.cc
diff --git a/ash/display/display_controller.cc b/ash/display/display_controller.cc
index 031a6b1d9c5d15a107f3938c2bb7249739f78b1c..0bb90b12c0d5fe235a958d0b14d35ddc2b091a9d 100644
--- a/ash/display/display_controller.cc
+++ b/ash/display/display_controller.cc
@@ -15,7 +15,7 @@
 #include "ash/display/mirror_window_controller.h"
 #include "ash/display/root_window_transformers.h"
 #include "ash/display/virtual_keyboard_window_controller.h"
-#include "ash/host/root_window_host_factory.h"
+#include "ash/host/window_tree_host_factory.h"
 #include "ash/root_window_controller.h"
 #include "ash/root_window_settings.h"
 #include "ash/screen_util.h"
@@ -707,7 +707,7 @@ aura::RootWindow* DisplayController::AddRootWindowForDisplay(
       GetDisplayManager()->GetDisplayInfo(display.id());
   const gfx::Rect& bounds_in_native = display_info.bounds_in_native();
   aura::RootWindow::CreateParams params(bounds_in_native);
-  params.host = Shell::GetInstance()->root_window_host_factory()->
+  params.host = Shell::GetInstance()->window_tree_host_factory()->
       CreateWindowTreeHost(bounds_in_native);
   aura::RootWindow* root_window = new aura::RootWindow(params);
   root_window->window()->SetName(