VR: Add a loading indicator to the scene.

chrome/browser/android/vr_shell/ui_elements/ui_element.h

Index: chrome/browser/android/vr_shell/ui_elements/ui_element.h
diff --git a/chrome/browser/android/vr_shell/ui_elements/ui_element.h b/chrome/browser/android/vr_shell/ui_elements/ui_element.h
index 2cf8a02491190c00a0184ea02213422419057d52..b0f980772e4a5f8cd4393f9818c09e9921239a77 100644
--- a/chrome/browser/android/vr_shell/ui_elements/ui_element.h
+++ b/chrome/browser/android/vr_shell/ui_elements/ui_element.h
@@ -223,6 +223,10 @@ class UiElement : public WorldRectangle {
   bool dirty() const { return dirty_; }
   void set_dirty(bool dirty) { dirty_ = dirty; }
 
+  // By default, sets an element to be visible or not. This may be overridden to
+  // allow finer control of element visibility.
+  virtual void SetEnabled(bool enabled);
+
  private:
   // Valid IDs are non-negative.
   int id_ = -1;