cleanup/ Remove unused forward declaration from app_window.h

BUG=

Review URL: https://codereview.chromium.org/1051253006

Cr-Commit-Position: refs/heads/master@{#325455}

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;