Introduce aura::Env controller to abstract away changes to aura::Env

ui/aura/window_event_dispatcher.h

Index: ui/aura/window_event_dispatcher.h
diff --git a/ui/aura/window_event_dispatcher.h b/ui/aura/window_event_dispatcher.h
index d54d10a3a8325fc71e3788ca9f0b8c349a37f3a9..75568b09d1494a90027ae73e9a2a7b9b6e1146e3 100644
--- a/ui/aura/window_event_dispatcher.h
+++ b/ui/aura/window_event_dispatcher.h
@@ -45,6 +45,7 @@ class TouchEvent;
 
 namespace aura {
 class TestScreen;
+class WindowEventDispatcherDelegate;
 class WindowTargeter;
 class WindowTreeHost;
 
@@ -272,6 +273,8 @@ class AURA_EXPORT WindowEventDispatcher : public ui::EventProcessor,
 
   bool transform_events_;
 
+  std::unique_ptr<WindowEventDispatcherDelegate> delegate_;
+
   // Used to schedule reposting an event.
   base::WeakPtrFactory<WindowEventDispatcher> repost_event_factory_;