Summary:
Flashing scroll indicators is a standard behavior on iOS to show the user there's more content.
Launch RNTester on iOS, go to the ScrollView section, tap the "Flash scroll indicators" button.
You'll see this:

I've exposed the method `flashScrollIndicators` on all scrolling components that were already exposing a `scrollToXXX` method so it's usable from those components using a ref.
Let me know what you think.
Closes https://github.com/facebook/react-native/pull/14058
Differential Revision: D5103239
Pulled By: shergin
fbshipit-source-id: caad8474fbe475065418d771b17e4ea9766ffcdc