Fixed crash during ScrollView initialisation if Platform.OS is not iOS or Android

Summary:
In case when Platform is different from Android or iOS, ScrollView initialised RCTScrollView two times, which caused a crash. It looks for me that default option is obsolete and can be united with iOS one to fix this issue.

## Changelog:
[Internal] [Fixed] - Fixed crash during ScrollView initialisation if Platform.OS is not iOS or Android

Reviewed By: ejanzer

Differential Revision: D19623046

fbshipit-source-id: 84f8a46ea24b463aa6aae761f4386ab7e4e95f9b