code: public float rotateToFaceCamera(final Widget widget) {
          final float yaw = getMainCameraRigYaw();
          GVRTransform t = getMainCameraRig().getHeadTransform();
          widget.rotateWithPivot(t.getRotationW(), 0, t.getRotationY(), 0, 0, 0, 0);
          return yaw;
      }
docs: Apply the necessary rotation to the transform so that it is in front of
  the camera. The actual rotation is performed not using the yaw angle but
  using equivalent quaternion values for better accuracy. But the yaw angle
  is still returned for backward compatibility.
  
  @param widget The transform to modify.
  @return The camera's yaw in degrees.