/// Called when this->getValPtr()->replaceAllUsesWith(new_value) is called,
/// _before_ any of the uses have actually been replaced. If WeakVH were
/// implemented as a CallbackVH, it would use this method to call
/// setValPtr(new_value). AssertingVH would do nothing in this method.
/// Called when this->getValPtr()->replaceAllUsesWith(new_value) is called,
/// _before_ any of the uses have actually been replaced. If WeakVH were
/// implemented as a CallbackVH, it would use this method to call
/// setValPtr(new_value). AssertingVH would do nothing in this method.