Summary:
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Explain the **motivation** for making this change. What existing problem does the pull request solve?
The problem occurs when a ScrollView's content height is smaller than the ScrollView height. If the method `scrollToEnd` is called on the ScrollView, it will pull the content down until the bottom of the content is aligned with the bottom of the Scrollview container.
This fix will ensure the proper functionality: That the furthest the ScrollView can scroll down is to where the top of the content container is at the origin (i.e., the ScrollView scroll number cannot be less than 0).
Prefer **small pull requests**. These are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.
**Test plan (required)**
Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI.
Make sure tests pass on both Travis and Circle CI.
I tested on a scenario where the ScrollView is almost the full size of the screen, and the content of the ScrollView has a height of much less. In this situation, the `scrollToEnd` method was executed and the content stayed in the same position. This is the intended behavior. If the content of the ScrollView is smaller than the height of the ScrollView, then the `scrollToEnd` method should not scroll anywhere.
**Code formatting**
Look around. Match the style of the rest of the codebase. See also the simple [style guide](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#style-guide).
For more info, see the ["Pull Requests" section of our "Contributing" guidelines](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests).
Closes https://github.com/facebook/react-native/pull/12889
Reviewed By: javache
Differential Revision: D5289894
Pulled By: sahrens
fbshipit-source-id: df2e779ee855c1dea85d33649d754371ad244bca
502604074c