[change] Linking.getInitialURL returns initial URL

Close #486

src/apis/Linking/index.js

@@ -2,6 +2,10 @@
  * @flow
  */
 
+import { canUseDOM } from 'fbjs/lib/ExecutionEnvironment';
+
+const initialURL = canUseDOM ? window.location.href : '';
+
 const Linking = {
   addEventListener() {},
   removeEventListener() {},
@@ -9,7 +13,7 @@ const Linking = {
     return Promise.resolve(true);
   },
   getInitialURL() {
-    return Promise.resolve('');
+    return Promise.resolve(initialURL);
   },
   openURL(url: string) {
     try {