Refactor ShellDesktopController and ShellNativeAppWindow to allow for non-aura implementations.

extensions/shell/browser/shell_app_window_client.h

Index: extensions/shell/browser/shell_app_window_client.h
diff --git a/extensions/shell/browser/shell_app_window_client.h b/extensions/shell/browser/shell_app_window_client.h
index 0ead4a81685a1676546a290dc3f4d68d07037876..ecb5f0ba3856cd09ec855c97b1ae02802371af38 100644
--- a/extensions/shell/browser/shell_app_window_client.h
+++ b/extensions/shell/browser/shell_app_window_client.h
@@ -20,6 +20,8 @@ class ShellAppWindowClient : public AppWindowClient {
   // AppWindowClient overrides:
   AppWindow* CreateAppWindow(content::BrowserContext* context,
                              const Extension* extension) override;
+  // Note that CreateNativeAppWindow is defined in separate (per-framework)
+  // implementation files.
   NativeAppWindow* CreateNativeAppWindow(
       AppWindow* window,
       AppWindow::CreateParams* params) override;