title: ScrollView.getChildAt(0).getHeight() can't get the right value
body: <p>I have a ScrollView which child's height is 500px,this contain 2
  button and the sum of height of them is 500px.but the value of
  child.getHeight() is 788 not 500 I know ScrollView extends Framelayout,i replace the ScrollView with  Framelayout in the same scene and the value of child.getHeight() is 500.</p>
  
  <p>Why is this happen?</p>