Adds a porting layer so aura can be made to work with mus

ui/aura/env.h

Index: ui/aura/env.h
diff --git a/ui/aura/env.h b/ui/aura/env.h
index 01b9e0ddaf45f286d8a301af4a3125803e6f45d6..d557a1f50f54d8f82d6276feaa940d017d1fb135 100644
--- a/ui/aura/env.h
+++ b/ui/aura/env.h
@@ -7,6 +7,7 @@
 
 #include <memory>
 
+#include "base/callback.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "base/supports_user_data.h"
@@ -28,6 +29,7 @@ class EnvTestHelper;
 class EnvObserver;
 class InputStateLookup;
 class Window;
+class WindowPort;
 class WindowTreeHost;
 
 // A singleton object that tracks general state within Aura.
@@ -35,10 +37,16 @@ class AURA_EXPORT Env : public ui::EventTarget, public base::SupportsUserData {
  public:
   ~Env() override;
 
-  static std::unique_ptr<Env> CreateInstance();
+  using WindowPortFactory =
+      base::Callback<std::unique_ptr<WindowPort>(Window* window)>;
+  static std::unique_ptr<Env> CreateInstance(
+      const WindowPortFactory& window_port_factory = WindowPortFactory());
   static Env* GetInstance();
   static Env* GetInstanceDontCreate();
 
+  // Called internally to create the appropriate WindowPort implementation.
+  std::unique_ptr<WindowPort> CreateWindowPort(Window* window);
+
   void AddObserver(EnvObserver* observer);
   void RemoveObserver(EnvObserver* observer);
 
@@ -71,7 +79,7 @@ class AURA_EXPORT Env : public ui::EventTarget, public base::SupportsUserData {
   friend class Window;
   friend class WindowTreeHost;
 
-  Env();
+  explicit Env(const WindowPortFactory& window_port_factory);
 
   void Init();
 
@@ -90,6 +98,8 @@ class AURA_EXPORT Env : public ui::EventTarget, public base::SupportsUserData {
   std::unique_ptr<ui::EventTargetIterator> GetChildIterator() const override;
   ui::EventTargeter* GetEventTargeter() override;
 
+  WindowPortFactory window_port_factory_;
+
   base::ObserverList<EnvObserver> observers_;
 
   int mouse_button_flags_;