[PR] ReactDOM takes optional `callback` in 3rd argument
[PR] ReactDOM takes optional `callback` in 3rd argument

Summary:
FIX: #4060

[`ReactDOM.render`](https://facebook.github.io/react/docs/react-dom.html#render) takes `callback` but this argument is missing in flow.
Closes https://github.com/facebook/flow/pull/4061

Differential Revision: D5200024

Pulled By: mroch

fbshipit-source-id: a75b7d74c4716aec7dbf05a9875f6ac87224cb09

MichaelDeBoey/flow,popham/flow,MichaelDeBoey/flow,samwgoldman/flow,TiddoLangerak/flow,facebook/flow,jamesgpearce/flow,JonathanUsername/flow,MichaelDeBoey/flow,MichaelDeBoey/flow,AgentME/flow,gabelevi/flow,popham/flow,claudiopro/flow,gabelevi/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,nmote/flow,TiddoLangerak/flow,fletcherw/flow,TiddoLangerak/flow,nmote/flow,MichaelDeBoey/flow,ylu1317/flow,nmote/flow,facebook/flow,nmote/flow,mroch/flow,fletcherw/flow,jamesgpearce/flow,jamesgpearce/flow,gabelevi/flow,ylu1317/flow,mroch/flow,mroch/flow,nmote/flow,popham/flow,jamesgpearce/flow,samwgoldman/flow,samwgoldman/flow,popham/flow,mroch/flow,AgentME/flow,gabelevi/flow,fletcherw/flow,gabelevi/flow,jamesgpearce/flow,TiddoLangerak/flow,mroch/flow,JonathanUsername/flow,AgentME/flow,popham/flow,samwgoldman/flow,gabelevi/flow,ylu1317/flow,claudiopro/flow,popham/flow,claudiopro/flow,ylu1317/flow,popham/flow,fletcherw/flow,JonathanUsername/flow,gabelevi/flow,claudiopro/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,JonathanUsername/flow,JonathanUsername/flow,facebook/flow,mroch/flow,fletcherw/flow,AgentME/flow,jamesgpearce/flow,samwgoldman/flow,nmote/flow,nmote/flow,TiddoLangerak/flow,AgentME/flow,ylu1317/flow,fletcherw/flow,facebook/flow,mroch/flow,facebook/flow,claudiopro/flow,JonathanUsername/flow