Clearer code in Button.js
Clearer code in Button.js

Summary:
Thanks for submitting a PR! Please read these instructions carefully:

- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] Match the **code formatting** of the rest of the codebase.
- [x] Target the `master` branch, NOT a "stable" branch.

What existing problem does the pull request solve?
* Combined repeating `if` checks for clearer logic.
* `defaultBlue` is inlined because it is only used once for each OS.

- Updated the `Button.js` file in an existing project and all buttons behave exactly the same. Here is a simple demo:
  ![](http://i.makeagif.com/media/6-11-2017/B0Bhry.gif)

- There is no change in the underlying logic of the code.
Closes https://github.com/facebook/react-native/pull/14455

Differential Revision: D5330112

Pulled By: hramos

fbshipit-source-id: 7fd1a0609f0bb2123208d0e1b3da2bc779f9583d

dikaiosune/react-native,facebook/react-native,jevakallio/react-native,hoangpham95/react-native,cdlewis/react-native,javache/react-native,exponent/react-native,myntra/react-native,jevakallio/react-native,brentvatne/react-native,cpunion/react-native,negativetwelve/react-native,javache/react-native,pandiaraj44/react-native,exponent/react-native,kesha-antonov/react-native,formatlos/react-native,thotegowda/react-native,kesha-antonov/react-native,hoastoolshop/react-native,pandiaraj44/react-native,myntra/react-native,frantic/react-native,farazs/react-native,formatlos/react-native,facebook/react-native,Bhullnatik/react-native,pandiaraj44/react-native,hammerandchisel/react-native,brentvatne/react-native,hammerandchisel/react-native,arthuralee/react-native,thotegowda/react-native,makadaw/react-native,brentvatne/react-native,ptmt/react-native-macos,frantic/react-native,makadaw/react-native,formatlos/react-native,farazs/react-native,mironiasty/react-native,hoangpham95/react-native,rickbeerendonk/react-native,cpunion/react-native,hoastoolshop/react-native,mironiasty/react-native,exponent/react-native,ptmt/react-native-macos,farazs/react-native,javache/react-native,hoangpham95/react-native,doochik/react-native,facebook/react-native,negativetwelve/react-native,arthuralee/react-native,ptmt/react-native-macos,cdlewis/react-native,kesha-antonov/react-native,mironiasty/react-native,Bhullnatik/react-native,myntra/react-native,pandiaraj44/react-native,javache/react-native,mironiasty/react-native,cdlewis/react-native,hoastoolshop/react-native,rickbeerendonk/react-native,dikaiosune/react-native,Bhullnatik/react-native,cpunion/react-native,hoangpham95/react-native,hammerandchisel/react-native,hoastoolshop/react-native,formatlos/react-native,facebook/react-native,formatlos/react-native,hammerandchisel/react-native,mironiasty/react-native,frantic/react-native,dikaiosune/react-native,doochik/react-native,doochik/react-native,cdlewis/react-native,brentvatne/react-native,myntra/react-native,exponent/react-native,formatlos/react-native,Bhullnatik/react-native,dikaiosune/react-native,myntra/react-native,negativetwelve/react-native,thotegowda/react-native,makadaw/react-native,farazs/react-native,facebook/react-native,jevakallio/react-native,mironiasty/react-native,thotegowda/react-native,myntra/react-native,thotegowda/react-native,ptmt/react-native-macos,janicduplessis/react-native,ptmt/react-native-macos,jevakallio/react-native,mironiasty/react-native,mironiasty/react-native,hammerandchisel/react-native,ptmt/react-native-macos,cpunion/react-native,javache/react-native,myntra/react-native,pandiaraj44/react-native,rickbeerendonk/react-native,cdlewis/react-native,hoastoolshop/react-native,makadaw/react-native,arthuralee/react-native,kesha-antonov/react-native,doochik/react-native,formatlos/react-native,kesha-antonov/react-native,cpunion/react-native,hammerandchisel/react-native,hoastoolshop/react-native,exponent/react-native,janicduplessis/react-native,pandiaraj44/react-native,exponent/react-native,kesha-antonov/react-native,farazs/react-native,javache/react-native,doochik/react-native,cdlewis/react-native,jevakallio/react-native,hoangpham95/react-native,Bhullnatik/react-native,javache/react-native,janicduplessis/react-native,hoastoolshop/react-native,kesha-antonov/react-native,doochik/react-native,cpunion/react-native,rickbeerendonk/react-native,jevakallio/react-native,arthuralee/react-native,negativetwelve/react-native,janicduplessis/react-native,facebook/react-native,farazs/react-native,dikaiosune/react-native,farazs/react-native,negativetwelve/react-native,exponent/react-native,makadaw/react-native,kesha-antonov/react-native,exponentjs/react-native,exponentjs/react-native,facebook/react-native,jevakallio/react-native,exponentjs/react-native,hoastoolshop/react-native,jevakallio/react-native,dikaiosune/react-native,exponentjs/react-native,frantic/react-native,exponent/react-native,hoangpham95/react-native,farazs/react-native,pandiaraj44/react-native,cpunion/react-native,myntra/react-native,negativetwelve/react-native,janicduplessis/react-native,thotegowda/react-native,rickbeerendonk/react-native,farazs/react-native,brentvatne/react-native,brentvatne/react-native,doochik/react-native,Bhullnatik/react-native,makadaw/react-native,exponentjs/react-native,makadaw/react-native,ptmt/react-native-macos,makadaw/react-native,arthuralee/react-native,rickbeerendonk/react-native,janicduplessis/react-native,formatlos/react-native,javache/react-native,kesha-antonov/react-native,doochik/react-native,Bhullnatik/react-native,thotegowda/react-native,makadaw/react-native,exponentjs/react-native,jevakallio/react-native,thotegowda/react-native,facebook/react-native,rickbeerendonk/react-native,rickbeerendonk/react-native,exponentjs/react-native,cdlewis/react-native,myntra/react-native,dikaiosune/react-native,formatlos/react-native,cpunion/react-native,hoangpham95/react-native,javache/react-native,negativetwelve/react-native,cdlewis/react-native,dikaiosune/react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