Summary:
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There's a positioning bug in `VirtualizedList` when `ItemSeparatorComponent` is defined for a list in horizontal or inverted mode. And also we face this bug in `FlatList`, because it is using `VirtualizedList` to render lists.
This commit will fix the [#15777](#15777).

Before fix:
```
<FlatList
  ...
  horizontal={true}
  inverted={true}
  ...
/>
```

![image](https://user-images.githubusercontent.com/15084663/30205251-95f14c70-949d-11e7-85e9-7a3304a52818.png)

```
<FlatList
  ...
  horizontal={true}
  inverted={false}
  ...
/>
```
![image](https://user-images.githubusercontent.com/15084663/30205411-f01d27b4-949d-11e7-991e-00aeae0c01e0.png)

I ran this code with all possible values of `horizontal` and `inverted` props in `FlatList` and `VirtualizedList` and the results of each run was as below:
After fix bug:

```
<FlatList
  ...
  horizontal={true}
  inverted={false}
  ...
/>
```
![image](https://user-images.githubusercontent.com/15084663/30205498-323bcf60-949e-11e7-8ba0-465614ea5cf2.png)

```
<FlatList
  ...
  horizontal={true}
  inverted={true}
  ...
/>
```
![image](https://user-images.githubusercontent.com/15084663/30205543-5274f612-949e-11e7-9660-bbdf8194cd27.png)
Closes #15865

Differential Revision: D5797266

Pulled By: hramos

fbshipit-source-id: 7d44fa797dbd9e83eb2bdd7833e9dd9707d9d822