Add proptypes for scrollview drag start & end handlers

Summary:
`ScrollView` has a bunch of `onFoo` handlers for scrolling-related events, most of which have a proptype defined and are documented. However, `onScrollBeginDrag` and `onScrollEndDrag` do not currently have a proptype and are not currently documented (as noted at https://stackoverflow.com/a/41793747/1709587). It seems reasonable to bring consistency and to provide documentation of these otherwise hard-to-discover props.

I haven't added or run any tests, and don't plan to do so (beyond waiting and seeing that no existing checks fail in CircleCI).

I have also created a PR to update the documentation at https://github.com/facebook/react-native-website/pull/99

*(None needed; this isn't a functionality change.)*
Closes https://github.com/facebook/react-native/pull/17368

Differential Revision: D6642695

Pulled By: TheSavior

fbshipit-source-id: fa40ed2ae6d5947a161b816a47441d8f5d4d9c4d

Libraries/Components/ScrollView/ScrollView.js

@@ -256,6 +256,15 @@ const ScrollView = createReactClass({
      * events can be controlled using the `scrollEventThrottle` prop.
      */
     onScroll: PropTypes.func,
+    /**
+     * Called when the user begins to drag the scroll view.
+     */
+    onScrollBeginDrag: PropTypes.func,
+    /**
+     * Called when the user stops dragging the scroll view and it either stops
+     * or begins to glide.
+     */
+    onScrollEndDrag: PropTypes.func,
     /**
      * Called when scrollable content view of the ScrollView changes.
      *