Avoid duplicate math when computing VR scene hierarchy.

chrome/browser/android/vr_shell/ui_elements.h

Index: chrome/browser/android/vr_shell/ui_elements.h
diff --git a/chrome/browser/android/vr_shell/ui_elements.h b/chrome/browser/android/vr_shell/ui_elements.h
index 69ed336d6ba8d3562ab6fe9b4f4b29752a659adb..e4a082ed1f2cd86b45bba6233336962e16eb0602 100644
--- a/chrome/browser/android/vr_shell/ui_elements.h
+++ b/chrome/browser/android/vr_shell/ui_elements.h
@@ -59,7 +59,7 @@ struct Transform {
 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;
@@ -154,6 +154,12 @@ struct ContentRectangle : public WorldRectangle {
 
   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);
 };