Fix SnapshotExample in rn-tester

Summary:
Changelog: [General][Fix] Fix SnapshotExample in rn-tester

Before this diff, the <Image> has no height. You can tell because the backgroundColor of <Image> is black but it doesn't show up on this surface.

Reviewed By: RSNara

Differential Revision: D33649085

fbshipit-source-id: 1b0fa0a82bce29923afee000d0828cbd90845abf

packages/rn-tester/js/examples/Snapshot/SnapshotExample.js

@@ -21,7 +21,7 @@ class ScreenshotExample extends React.Component<{...}, $FlowFixMeState> {
 
   render() {
     return (
-      <View>
+      <View style={style.container}>
         <Text onPress={this.takeScreenshot} style={style.button}>
           Click to take a screenshot
         </Text>
@@ -38,13 +38,15 @@ class ScreenshotExample extends React.Component<{...}, $FlowFixMeState> {
 }
 
 const style = StyleSheet.create({
+  container: {
+    flex: 1,
+  },
   button: {
     marginBottom: 10,
     fontWeight: '500',
   },
   image: {
     flex: 1,
-    height: 300,
     resizeMode: 'contain',
     backgroundColor: 'black',
   },