Avoid duplicate math when computing VR scene hierarchy.

chrome/browser/android/vr_shell/ui_elements.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_ELEMENTS_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 41 matching lines @@
   void Rotate(float ax, float ay, float az, float rad);
   void Translate(float tx, float ty, float tz);
   void Scale(float sx, float sy, float sz);
 
   gvr::Mat4f to_world;
 };
 
 class WorldRectangle {
  public:
   const gvr::Mat4f& TransformMatrix() const;
-  void SetTransform(const Transform& transform);
+  Transform* mutable_transform() { return &transform_; }
 
   gvr::Vec3f GetCenter() const;
   gvr::Vec3f GetNormal() const;
 
   // Computes the distance from |ray_origin| to this rectangles's plane, along
   // |ray_vector|. Returns true and populates |distance| if the calculation is
   // possible, and false if the ray is parallel to the plane.
   bool GetRayDistance(const gvr::Vec3f& ray_origin,
                       const gvr::Vec3f& ray_vector,
                       float* distance) const;
@@ skipping 74 matching lines @@
 
   Fill fill = Fill::NONE;
 
   Colorf edge_color = {1.0f, 1.0f, 1.0f, 1.0f};
   Colorf center_color = {1.0f, 1.0f, 1.0f, 1.0f};
 
   int gridline_count = 1;
 
   int draw_phase = 1;
 
+  // This transform can be used by children to derive position of its parent.
+  Transform inheritable_transform;
+
+  // A flag usable during transformation calculates to avoid duplicate work.
+  bool dirty;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(ContentRectangle);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_H_