Implement a postMessage function and an onMessage event for webviews …

Summary:
JS API very similar to web workers and node's child process.

Work has been done by somebody else for the Android implementation over at #7020, so we'd need to have these in sync before anything gets merged.

I've made a prop `messagingEnabled` to be more explicit about creating globals—it might be sufficient to just check for an onMessage handler though.

![screen shot 2016-09-06 at 10 28 23](https://cloud.githubusercontent.com/assets/7275322/18268669/b1a12348-741c-11e6-91a1-ad39d5a8bc03.png)
Closes https://github.com/facebook/react-native/pull/9762

Differential Revision: D4008260

fbshipit-source-id: 84b1afafbc0ab1edc3dfbf1a8fb870218e171a4c

Libraries/Components/WebView/WebView.android.js

@@ -58,6 +58,7 @@ class WebView extends React.Component {
     automaticallyAdjustContentInsets: PropTypes.bool,
     contentInset: EdgeInsetsPropType,
     onNavigationStateChange: PropTypes.func,
+    onMessage: PropTypes.func,
     onContentSizeChange: PropTypes.func,
     startInLoadingState: PropTypes.bool, // force WebView to show loadingView on first load
     style: View.propTypes.style,
@@ -218,6 +219,8 @@ class WebView extends React.Component {
         userAgent={this.props.userAgent}
         javaScriptEnabled={this.props.javaScriptEnabled}
         domStorageEnabled={this.props.domStorageEnabled}
+        messagingEnabled={typeof this.props.onMessage === 'function'}
+        onMessage={this.onMessage}
         contentInset={this.props.contentInset}
         automaticallyAdjustContentInsets={this.props.automaticallyAdjustContentInsets}
         onContentSizeChange={this.props.onContentSizeChange}
@@ -268,6 +271,14 @@ class WebView extends React.Component {
     );
   };
 
+  postMessage = (data) => {
+    UIManager.dispatchViewManagerCommand(
+      this.getWebViewHandle(),
+      UIManager.RCTWebView.Commands.postMessage,
+      [String(data)]
+    );
+  };
+
   /**
    * We return an event with a bunch of fields including:
    *  url, title, loading, canGoBack, canGoForward
@@ -310,9 +321,18 @@ class WebView extends React.Component {
     });
     this.updateNavigationState(event);
   };
+
+  onMessage = (event: Event) => {
+    var {onMessage} = this.props;
+    onMessage && onMessage(event);
+  }
 }
 
-var RCTWebView = requireNativeComponent('RCTWebView', WebView);
+var RCTWebView = requireNativeComponent('RCTWebView', WebView, {
+  nativeOnly: {
+    messagingEnabled: PropTypes.bool,
+  },
+});
 
 var styles = StyleSheet.create({
   container: {