Fix for: You cannot blur a text-input if you tab to it. (#27038)

Summary:
I've been working on a new iOS experience with lots of text inputs and this has been driving me a bit nuts…

If you're in a scrollview with `keyboardShouldPersistTaps="handled"` and you tab through your text-inputs, you aren't able to tap outside of a given text-input to blur it (and dismiss the keyboard).

I wrote up a quick explanation and some repo steps here: https://snack.expo.io/BJBcKgrqB

The patch i came up with, after poking around for a little bit seems terrifying - so almost certainly not it. But if it's helpful at all - decided to just got ahead and submit it.

## Changelog

[iOS] [Fixed] - TextInput blur when tabbing in iOS simulator.
Pull Request resolved: #27038

Test Plan:
I tried to think of a way to test this in jest… but i didn't get very far sorry 😢

I did create a snack here so you can demo the issue: https://snack.expo.io/BJBcKgrqB

I also created two videos…

**Here's the text input not working when i try to blur it after tabbing in simulator**
![ezgif-1-dc85b405c760](https://user-images.githubusercontent.com/169705/67726290-8e5d2d00-f9a2-11e9-98c9-29c0b0bb25b9.gif)

**Here's it working after I applied this patch**
![ezgif-1-ed9f6b19653d](https://user-images.githubusercontent.com/169705/67726309-97e69500-f9a2-11e9-9992-c2a79d0bd408.gif)

Thanks!

Differential Revision: D18262867

Pulled By: TheSavior

fbshipit-source-id: 4087f3a27a7e6a146f7f84d7c6e9e8e2b6adc75d

Libraries/Components/TextInput/TextInput.js

@@ -1095,6 +1095,7 @@ const TextInput = createReactClass({
   },
 
   _onFocus: function(event: FocusEvent) {
+    this.focus();
     if (this.props.onFocus) {
       this.props.onFocus(event);
     }