Add missing content quad element to native scene manager.

chrome/browser/android/vr_shell/ui_scene_manager.h

Index: chrome/browser/android/vr_shell/ui_scene_manager.h
diff --git a/chrome/browser/android/vr_shell/ui_scene_manager.h b/chrome/browser/android/vr_shell/ui_scene_manager.h
index 529d047da3d5b31315ca31b91cabb118f5a7c7fc..e7b935804da6a57bff840336207779d38abb882d 100644
--- a/chrome/browser/android/vr_shell/ui_scene_manager.h
+++ b/chrome/browser/android/vr_shell/ui_scene_manager.h
@@ -34,6 +34,7 @@ class UiSceneManager {
   // 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;