[react-native] Make document.js into a polyfill. Fixes #1149

Summary:
@public
document shimming must run before anything else. However, we don't currently guarantee that. This moves the document shimming into `document.js` which is used as a polyfill.

Test Plan:
* start server
* go to playground app
* require `NativeModules` as the first thing
* open chrome debugger
* no error

packager.js

@@ -200,7 +200,12 @@ function getAppMiddleware(options) {
     cacheVersion: '2',
     transformModulePath: require.resolve('./transformer.js'),
     assetRoots: options.assetRoots,
-    assetExts: ['png', 'jpeg', 'jpg']
+    assetExts: ['png', 'jpeg', 'jpg'],
+    polyfillModuleNames: [
+      require.resolve(
+        '../Libraries/JavaScriptAppEngine/polyfills/document.js'
+      ),
+    ],
   });
 }