Remove iOS platform check for running devtools

Summary:
Currently, DevTools only work under ios (although this is undocumented!), because the JavaScriptEngine initialization process skips setupDevTools() on android.

DevTools work fine with Android, as tested on 0.26, 0.27, and 0.28 using Nuclide's inspector.

For reference, [the relevant issue on react-devtools](https://github.com/facebook/react-devtools/issues/229).
Closes https://github.com/facebook/react-native/pull/8095

Reviewed By: bestander

Differential Revision: D3430206

Pulled By: javache

fbshipit-source-id: 826f0252fff3f0ec2c843149540a0109e34d1d18

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -213,7 +213,7 @@ function setUpMapAndSet() {
 function setUpDevTools() {
   if (__DEV__) {
     // not when debugging in chrome
-    if (!window.document && require('Platform').OS === 'ios') {
+    if (!window.document) {
       const setupDevtools = require('setupDevtools');
       setupDevtools();
     }