MacViewsBrowser: Provide a stub for ChromeExtensionChooserDialog::ShowDialog()

chrome/browser/extensions/chrome_extension_chooser_dialog.h

 // Copyright 2016 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 CHROME_BROWSER_EXTENSIONS_CHROME_EXTENSION_CHOOSER_DIALOG_H_
 #define CHROME_BROWSER_EXTENSIONS_CHROME_EXTENSION_CHOOSER_DIALOG_H_
 
 #include "base/macros.h"
 #include "build/build_config.h"
 
 class ChooserController;
 
 namespace content {
 class WebContents;
 }
 
 class ChromeExtensionChooserDialog {
  public:
   explicit ChromeExtensionChooserDialog(content::WebContents* web_contents)
       : web_contents_(web_contents) {}
   ~ChromeExtensionChooserDialog() {}
 
   content::WebContents* web_contents() const { return web_contents_; }
 
-// TODO(juncai): remove this preprocessor directive once the non-Mac
-// implementation is done.
-#if defined(OS_MACOSX)
   void ShowDialog(ChooserController* chooser_controller) const;
-#endif  // defined(OS_MACOSX)
 
  private:
   content::WebContents* web_contents_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeExtensionChooserDialog);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_CHROME_EXTENSION_CHOOSER_DIALOG_H_