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

extensions/shell/browser/shell_app_window_client.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef EXTENSIONS_SHELL_BROWSER_SHELL_APP_WINDOW_CLIENT_H_
 #define EXTENSIONS_SHELL_BROWSER_SHELL_APP_WINDOW_CLIENT_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "extensions/browser/app_window/app_window_client.h"
 
 namespace extensions {
 
 // app_shell's AppWindowClient implementation.
 class ShellAppWindowClient : public AppWindowClient {
  public:
   ShellAppWindowClient();
   ~ShellAppWindowClient() override;
 
   // 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;
   void OpenDevToolsWindow(content::WebContents* web_contents,
                           const base::Closure& callback) override;
   bool IsCurrentChannelOlderThanDev() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ShellAppWindowClient);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_SHELL_BROWSER_SHELL_APP_WINDOW_CLIENT_H_