Flow strict ScrollViewMock (facebook#22103)

Summary:
Related to facebook#22100

Turn Flow strict mode on for ScrollViewMock.
This file used to declare jest var as `any` but jest module is already typed in root flow folder.

Note: I had to use a quick fix for polyfillPromise. See here facebook#22101

- All flow tests succeed.

[GENERAL] [ENHANCEMENT] [ScrollViewMock.js] - Flow strict mode
Pull Request resolved: facebook#22103

Differential Revision: D12918380

Pulled By: TheSavior

fbshipit-source-id: cd3aba47b1a43e76a7da09e15cc2d9cfcdf7f56d

Libraries/Components/ScrollView/__mocks__/ScrollViewMock.js

@@ -5,15 +5,13 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
- * @flow
+ * @flow strict-local
  */
 
 /* eslint-env jest */
 
 'use strict';
 
-declare var jest: any;
-
 const React = require('React');
 const View = require('View');