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

ui/aura/window_tree_host_platform.h

Index: ui/aura/window_tree_host_platform.h
diff --git a/ui/aura/window_tree_host_platform.h b/ui/aura/window_tree_host_platform.h
index eb25d34955027eb50747e7219d1f223b9ee280de..0ee8824d2ebb5e36c903699fd3de6745ab084698 100644
--- a/ui/aura/window_tree_host_platform.h
+++ b/ui/aura/window_tree_host_platform.h
@@ -17,6 +17,8 @@
 
 namespace aura {
 
+class WindowPort;
+
 // The unified WindowTreeHost implementation for platforms
 // that implement PlatformWindow.
 class AURA_EXPORT WindowTreeHostPlatform
@@ -42,6 +44,8 @@ class AURA_EXPORT WindowTreeHostPlatform
 
  protected:
   WindowTreeHostPlatform();
+  explicit WindowTreeHostPlatform(std::unique_ptr<WindowPort> window_port);
+
   void SetPlatformWindow(std::unique_ptr<ui::PlatformWindow> window);
   ui::PlatformWindow* platform_window() { return window_.get(); }