Summary:
Changelog: [iOS][Internal] - Add iOS implementation of the button property of the PointerEvent object
This implements the `button` property on the PointerEvent object by storing the button which caused the down event in the `ActiveTouch` and reporting that button through `pointerdown` and `pointerup` events and -1 on all others. This diff also includes a small fix to the `pressure` property which was introduced due to `button` being correctly implemented.
Reviewed By: yungsters
Differential Revision: D38632543
fbshipit-source-id: 9dbbb23a9251f2e661faf37fdf206b9f0b26bc5f
92e4ed6a28