Fix react-native-web support for #3526 (#3546)

* MaskedViewIOS use broke react-native-web support, this fixes it.

* Fallback more gracefully.

* Actually return the value ...

src/PlatformHelpers.native.js

@@ -1,8 +1,9 @@
 import {
   BackAndroid as DeprecatedBackAndroid,
   BackHandler as ModernBackHandler,
+  MaskedViewIOS,
 } from 'react-native';
 
 const BackHandler = ModernBackHandler || DeprecatedBackAndroid;
 
-export { BackHandler };
+export { BackHandler, MaskedViewIOS };

src/PlatformHelpers.web.js

@@ -1,3 +1,6 @@
-import { BackHandler } from 'react-native';
+import React from 'react';
+import { BackHandler, View } from 'react-native';
 
-export { BackHandler };
+const MaskedViewIOS = () => <View>{this.props.children}</View>;
+
+export { BackHandler, MaskedViewIOS };

src/views/Header/Header.js

@@ -6,10 +6,10 @@ import {
   Image,
   Platform,
   StyleSheet,
-  MaskedViewIOS,
   View,
   ViewPropTypes,
 } from 'react-native';
+import { MaskedViewIOS } from '../../PlatformHelpers';
 import SafeAreaView from 'react-native-safe-area-view';
 
 import HeaderTitle from './HeaderTitle';
@@ -379,7 +379,7 @@ class Header extends React.PureComponent {
     if (
       options.headerLeft ||
       options.headerBackImage ||
-      Platform.OS === 'android' ||
+      Platform.OS !== 'ios' ||
       transitionPreset !== 'uikit'
     ) {
       return (