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: javache

Differential Revision: D3443980

Pulled By: andreicoman11

fbshipit-source-id: 3d7b2e83cf4158a1228d2e21510509ab63411a5d

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -220,7 +220,7 @@ function setUpCollections(): void {
 function setUpDevTools(): void {
   if (__DEV__) {
     // not when debugging in chrome
-    if (!window.document && require('Platform').OS === 'ios') {
+    if (!window.document) {
       const setupDevtools = require('setupDevtools');
       setupDevtools();
     }