title: scrollToLocation is undefined in sectionlist ref of react native, neither in flatlist
body: <p>I have tried Flatlist, SectionList, Animated.Flatlist and Animated.SectionList,
  Here is a simple example how to check scrollToLocation:</p>
  
  <pre><code>&lt;SectionList 
       onLayout={this.onLayout.bind(this)}
       ref={ref =&gt; {
       this.sectionList = ref
       console.log('ref.scrollToLocation: ' + CircularJSON.stringify(ref.scrollToLocation))
            } }
       ListHeaderComponent={() =&gt; &lt;View style={{height:300, width:300, backgroundColor:'green'}}&gt;&lt;/View&gt;}
       sections = {[{key: 1, data: [1,1,1,1,1,1,1,1,1,1,1,1,1,1,1]}]}
       renderItem = {() =&gt; &lt;View style={{width:300, height: 100, marginBottom:20, backgroundColor:'red'}}&gt;&lt;/View&gt;}
          /&gt;
  </code></pre>
  
  <p>Expected to see scrollToLocation method but showing <strong><em><code>undefined</code></em></strong>.
  Please help me if you have any idea. Thanks!</p>
  
  <pre><code>Env:
  "react": "16.6.3",
  "react-native": "0.57.8",
  Simulator: Xcode 10 iPhone X
  </code></pre>