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

Examples/UIExplorer/js/WebViewExample.js

@@ -217,6 +217,53 @@ class ScaledWebView extends React.Component {
   }
 }
 
+class MessagingTest extends React.Component {
+  webview = null
+
+  state = {
+    messagesReceivedFromWebView: 0,
+    message: '',
+  }
+
+  onMessage = e => this.setState({
+    messagesReceivedFromWebView: this.state.messagesReceivedFromWebView + 1,
+    message: e.nativeEvent.data,
+  })
+
+  postMessage = () => {
+    if (this.webview) {
+      this.webview.postMessage('"Hello" from React Native!');
+    }
+  }
+
+  render(): ReactElement<any> {
+    const {messagesReceivedFromWebView, message} = this.state;
+
+    return (
+      <View style={[styles.container, { height: 200 }]}>
+        <View style={styles.container}>
+          <Text>Messages received from web view: {messagesReceivedFromWebView}</Text>
+          <Text>{message || '(No message)'}</Text>
+          <View style={styles.buttons}>
+            <Button text="Send Message to Web View" enabled onPress={this.postMessage} />
+          </View>
+        </View>
+        <View style={styles.container}>
+          <WebView
+            ref={webview => { this.webview = webview; }}
+            style={{
+              backgroundColor: BGWASH,
+              height: 100,
+            }}
+            source={require('./messagingtest.html')}
+            onMessage={this.onMessage}
+          />
+        </View>
+      </View>
+    );
+  }
+}
+
 var styles = StyleSheet.create({
   container: {
     flex: 1,
@@ -391,5 +438,9 @@ exports.examples = [
         />
       );
     }
+  },
+  {
+    title: 'Mesaging Test',
+    render(): ReactElement<any> { return <MessagingTest />; }
   }
 ];