Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to ash.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index b98378d26e4be872f1e152da2a378265036a5b37..ca2fbfe0a9d35c2b28ed4b3d85c2299d7f8fbf6b 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -90,7 +90,6 @@ class NestedDispatcherController;
 class NewWindowDelegate;
 class PartialMagnificationController;
 class PowerButtonController;
-class WindowTreeHostFactory;
 class ScreenAsh;
 class SessionStateDelegate;
 class Shelf;
@@ -524,10 +523,6 @@ class ASH_EXPORT Shell
   }
 #endif  // defined(OS_CHROMEOS)
 
-  WindowTreeHostFactory* window_tree_host_factory() {
-    return window_tree_host_factory_.get();
-  }
-
   ShelfModel* shelf_model() {
     return shelf_model_.get();
   }
@@ -668,7 +663,6 @@ class ASH_EXPORT Shell
   scoped_ptr<internal::EventClientImpl> event_client_;
   scoped_ptr<internal::EventTransformationHandler>
       event_transformation_handler_;
-  scoped_ptr<WindowTreeHostFactory> window_tree_host_factory_;
 
   // An event filter that pre-handles key events while the partial
   // screenshot UI or the keyboard overlay is active.