Control the VrShell content quad via the HTML UI.

chrome/browser/android/vr_shell/ui_scene.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_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_SCENE_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 48 matching lines @@
   // arbitrary, but must match the unit used in animations.
   void UpdateTransforms(float screen_tilt, int64_t time_in_micro);
 
   // Handle a batch of commands passed from the UI HTML.
   void HandleCommands(const base::ListValue* commands, int64_t time_in_micro);
 
   const std::vector<std::unique_ptr<ContentRectangle>>& GetUiElements() const;
 
   ContentRectangle* GetUiElementById(int element_id);
 
+  ContentRectangle* GetContentQuad();
+
   // Return a monotonic time in microseconds for coordinating animations.
   static int64_t TimeInMicroseconds();
 
  private:
   void ApplyRecursiveTransforms(const ContentRectangle& element,
                                 ReversibleTransform* transform);
   void ApplyDictToElement(const base::DictionaryValue& dict,
                           ContentRectangle *element);
+
   std::vector<std::unique_ptr<ContentRectangle>> ui_elements_;
+  ContentRectangle* content_element_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(UiScene);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_SCENE_H_