title: EventListener in react native webview
body: <p>I am trying to get a scroll event back to react from a webview.
  According to the doc it seems that the way to go is using postMessage and injectedJavascript.</p>
  
  <p>Here is my code</p>
  
  <pre><code>  &lt;CustomWebView
      injectedJavaScript="window.addEventListener('scroll', function(event) {window.ReactNativeWebView.postMessage(JSON.stringify(event));});true;"
      onMessage={event =&gt; {
        console.log(event.nativeEvent.data);
      }}
      applicationNameForUserAgent="App"
      ignoreSslError
      useWebKit
      source={{ uri: net.uri }}
      startInLoadingState
    /&gt;
  </code></pre>
  
  <p>The onMessage part seems to be doing something because I am getting this in the logs for every scroll event, however no direction</p>
  
  <pre><code>{"isTrusted":true}
  </code></pre>
  
  <p>What have I missed?</p>