SnapshotViewIOS: Remove PropTypes (#21294)
SnapshotViewIOS: Remove PropTypes (#21294)

Summary:
Part of: https://github.com/react-native-community/discussions-and-proposals/issues/29

This PR removes all PropTypes from `SnapshotViewIOS`, and fills out the flow types for its event callbacks.
Pull Request resolved: https://github.com/facebook/react-native/pull/21294

Differential Revision: D10011659

Pulled By: TheSavior

fbshipit-source-id: 28bfa0ab58c0655f9b905d3cb6530b57166c67f9

exponentjs/react-native,pandiaraj44/react-native,hoangpham95/react-native,exponentjs/react-native,hoangpham95/react-native,javache/react-native,myntra/react-native,facebook/react-native,hammerandchisel/react-native,facebook/react-native,hammerandchisel/react-native,exponent/react-native,javache/react-native,exponent/react-native,javache/react-native,arthuralee/react-native,hoangpham95/react-native,javache/react-native,exponent/react-native,hammerandchisel/react-native,hoangpham95/react-native,janicduplessis/react-native,janicduplessis/react-native,exponentjs/react-native,pandiaraj44/react-native,exponentjs/react-native,facebook/react-native,pandiaraj44/react-native,facebook/react-native,hoangpham95/react-native,myntra/react-native,facebook/react-native,exponent/react-native,myntra/react-native,exponent/react-native,hammerandchisel/react-native,hammerandchisel/react-native,javache/react-native,myntra/react-native,exponent/react-native,hammerandchisel/react-native,facebook/react-native,pandiaraj44/react-native,arthuralee/react-native,myntra/react-native,arthuralee/react-native,hoangpham95/react-native,janicduplessis/react-native,exponent/react-native,hammerandchisel/react-native,exponentjs/react-native,pandiaraj44/react-native,javache/react-native,exponentjs/react-native,janicduplessis/react-native,hoangpham95/react-native,janicduplessis/react-native,arthuralee/react-native,myntra/react-native,hammerandchisel/react-native,janicduplessis/react-native,javache/react-native,arthuralee/react-native,javache/react-native,facebook/react-native,exponentjs/react-native,exponent/react-native,exponentjs/react-native,janicduplessis/react-native,pandiaraj44/react-native,pandiaraj44/react-native,javache/react-native,facebook/react-native,myntra/react-native,myntra/react-native,hoangpham95/react-native,facebook/react-native,pandiaraj44/react-native,janicduplessis/react-native,myntra/react-native