Summary:
* Added a default value for HMRLoadingView this will prevent errors from being thrown when `HMRClient` attempts to include it in a `web` context.
* [Web] [Added] - HMRLoadingView
Pull Request resolved: https://github.com/facebook/react-native/pull/23389
Differential Revision: D14045475
Pulled By: cpojer
fbshipit-source-id: df5c3cf3536af3b37eaf82342b6346bc25054319