Fix scalesPageToFit default property for iOS WebView.

Summary:
`scalesPageToFit`'s [documented default value is true](https://github.com/facebook/react-native/blob/master/Libraries/Components/WebView/WebView.ios.js#L318), but it's not explicitly set anywhere on iOS. On Android it's [set to true in defaultProps](https://github.com/facebook/react-native/blob/master/Libraries/Components/WebView/WebView.android.js#L203). This pull-request add the default behaviour.

```
import { WebView } from 'react-native;
/* ... */
<WebView source={{ uri: 'https://google.com' }} />
```

You should be able to zoom in and out using the pinch gesture.

1. Does the reviewer have any additional thoughts around the rest of the defaultProps which aren't explicitly set?
2. Does the reviewer have any additional thoughts on how these two components (iOS/ Android) can share more code?
Closes #14363

Differential Revision: D5345883

Pulled By: shergin

fbshipit-source-id: e7b9b89f13d6adcdc705244871e00b7a5da52c64

Libraries/Components/WebView/WebView.ios.js

@@ -367,6 +367,10 @@ class WebView extends React.Component {
     ]),
   };
 
+  static defaultProps = {
+    scalesPageToFit: true,
+  };
+
   state = {
     viewState: WebViewState.IDLE,
     lastErrorEvent: (null: ?ErrorEvent),