title: ViewTreeObserver does not call onScrollChanged after adding OnScrollChangedListener
body: <p>I have a layout with a webview and a button. The button is visible but becomes visible once the webview is scrolled. I'm using ViewTreeObserver, but for some reason, onScrollChanged is not called. Am I misunderstanding how onScrollChanged is called?</p>
  
  <pre><code>this.licenseAgreementView = (WebView) findViewById(R.id.licenseAgreementView);
  ViewTreeObserver observer = this.licenseAgreementView.getViewTreeObserver();
  observer.addOnScrollChangedListener(new OnScrollChangedListener() {
      public void onScrollChanged() {
      makeButtonVisible();
      }
  });
  </code></pre>