As of now, VrShellGl is tightly coupled to the parts of the skia UI.  We're moving towards having a vr::Ui abstract class, and VrShellGl should be able to render the UI using a clean interface to this class.

Ui object getters like the following should disappear:

  UiScene* scene() { return scene_.get(); }
  VrShellRenderer* vr_shell_renderer() { return vr_shell_renderer_.get(); }
  UiRenderer* ui_renderer() { return ui_renderer_.get(); }
  UiInputManager* input_manager() { return input_manager_.get(); }