Add missing content quad element to native scene manager.

chrome/browser/android/vr_shell/ui_scene_manager.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_ANDROID_VR_SHELL_UI_SCENE_MANAGER_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_SCENE_MANAGER_H_
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/timer/timer.h"
@@ skipping 16 matching lines @@
 
  private:
   void ConfigureSecurityWarnings();
   void OnSecurityWarningTimer();
 
   UiScene* scene_;
 
   // UI elemenet pointers (not owned by the scene manager).
   UiElement* permanent_security_warning_ = nullptr;
   UiElement* transient_security_warning_ = nullptr;
+  UiElement* main_content_ = nullptr;
 
   bool web_vr_mode_ = false;
   bool secure_origin_ = false;
 
   base::OneShotTimer security_warning_timer_;
 
   base::WeakPtrFactory<UiSceneManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UiSceneManager);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_SCENE_MANAGER_H_