Summary:
This PR fixes#15006 by removing all UI API calls from RCTScrollEvent.
`-[RCTScrollEvent arguments]` can now be called from a background thread.
The Main Thread Checker of Xcode 9 will not any longer produce runtime issues when calling this method.
1. create a React Native (version: this PR) project with a scroll view
2. open it in Xcode 9
3. launch it
4. scroll the scroll view
5. observe the runtime issues in Xcode. There should not contain "UI API called from background thread"-issues.
I verified my changes on this branch: https://github.com/HeEAaD/Demo-ReactNative-UI-not-on-main-thread/tree/fix
<!--
Thank you for sending the PR!
If you changed any code, please provide us with clear instructions on how you verified your changes work. In other words, a test plan is *required*. Bonus points for screenshots and videos!
Please read the Contribution Guidelines at https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md to learn more about contributing to React Native.
Happy contributing!
-->
Closes https://github.com/facebook/react-native/pull/15008
Differential Revision: D5424734
Pulled By: shergin
fbshipit-source-id: 56beec2d7603ea6782d55622567509f3758a4517
048a9ab10c