feat(new prop): containerStyle (#912)

* feat(new prop): containerStyle

* chore(docs): Add documentation for `containerStyle`

docs/Reference.md

 - [`onShouldStartLoadWithRequest`](Reference.md#onshouldstartloadwithrequest)
 - [`startInLoadingState`](Reference.md#startinloadingstate)
 - [`style`](Reference.md#style)
+- [`containerStyle`](Reference.md#containerStyle)
 - [`decelerationRate`](Reference.md#decelerationrate)
 - [`domStorageEnabled`](Reference.md#domstorageenabled)
 - [`javaScriptEnabled`](Reference.md#javascriptenabled)
 ### `onHttpError`
 
 Function that is invoked when the `WebView` receives an http error.
+
 > **_Note_**
 > Android API minimum level 23.
 
   source={{ uri: 'https://facebook.github.io/react-native' }}
   onHttpError={syntheticEvent => {
     const { nativeEvent } = syntheticEvent;
-    console.warn('WebView received error status code: ', nativeEvent.statusCode);
+    console.warn(
+      'WebView received error status code: ',
+      nativeEvent.statusCode,
+    );
   }}
 />
 ```
   onContentProcessDidTerminate={syntheticEvent => {
     const { nativeEvent } = syntheticEvent;
     console.warn('Content process terminated, reloading', nativeEvent);
-    this.refs.webview.reload()
+    this.refs.webview.reload();
   }}
 />
 ```
 
 ---
 
+### `containerStyle`
+
+A style object that allow you to customize the `WebView` container style. Please note that there are default styles (example: you need to add `flex: 0` to the style if you want to use `height` property).
+
+| Type  | Required |
+| ----- | -------- |
+| style | No       |
+
+Example:
+
+```jsx
+<WebView
+  source={{ uri: 'https://facebook.github.io/react-native' }}
+  containerStyle={{ marginTop: 20 }}
+/>
+```
+
+---
+
 ### `decelerationRate`
 
 A floating-point number that determines how quickly the scroll view decelerates after the user lifts their finger. You may also use the string shortcuts `"normal"` and `"fast"` which match the underlying iOS settings for `UIScrollViewDecelerationRateNormal` and `UIScrollViewDecelerationRateFast` respectively:

src/WebView.android.tsx

       renderLoading,
       source,
       style,
+      containerStyle,
       nativeConfig = {},
       ...otherProps
     } = this.props;
     }
 
     const webViewStyles = [styles.container, styles.webView, style];
+    const webViewContainerStyle = [styles.container, containerStyle];
 
     if (source && 'method' in source) {
       if (source.method === 'POST' && source.headers) {
     );
 
     return (
-      <View style={styles.container}>
+      <View style={webViewContainerStyle}>
         {webView}
         {otherView}
       </View>

src/WebView.ios.tsx

       renderError,
       renderLoading,
       style,
+      containerStyle,
       ...otherProps
     } = this.props;
 
     }
 
     const webViewStyles = [styles.container, styles.webView, style];
+    const webViewContainerStyle = [styles.container, containerStyle];
 
     const onShouldStartLoadWithRequest = createOnShouldStartLoadWithRequest(
       this.onShouldStartLoadWithRequestCallback,
     );
 
     return (
-      <View style={styles.container}>
+      <View style={webViewContainerStyle}>
         {webView}
         {otherView}
       </View>

src/WebViewTypes.ts

 import {
   NativeSyntheticEvent,
   ViewProps,
+  StyleProp,
+  ViewStyle,
   NativeMethodsMixin,
   Constructor,
   UIManagerStatic,
    */
   javaScriptEnabled?: boolean;
 
+  /**
+   * Stylesheet object to set the style of the container view.
+   */
+  containerStyle?: StyleProp<ViewStyle>;
+
   /**
    * Function that returns a view to show if there's an error.
    */