cleanup/ Remove unused forward declaration from app_window.h

extensions/browser/app_window/app_window.h

Index: extensions/browser/app_window/app_window.h
diff --git a/extensions/browser/app_window/app_window.h b/extensions/browser/app_window/app_window.h
index 6c0e1358f6c152aeb39a6d9c3d0043efb04f921a..e9679dc1b2fea0979aa5337a03b621ba19eeafe2 100644
--- a/extensions/browser/app_window/app_window.h
+++ b/extensions/browser/app_window/app_window.h
@@ -33,19 +33,13 @@ class BrowserContext;
 class WebContents;
 }
 
-namespace ui {
-class BaseWindow;
-}
-
 namespace extensions {
 
 class AppDelegate;
 class AppWebContentsHelper;
 class Extension;
-class ExtensionRegistry;
 class NativeAppWindow;
 class PlatformAppBrowserTest;
-class WindowController;
 
 struct DraggableRegion;