Only update composited-scrolling state once after layout.

LayoutTests/compositing/overflow/resources/automatically-opt-into-composited-scrolling.js

 var debugMode = false;
 
 if (window.testRunner)
   testRunner.dumpAsText();
 
 function write(str)
 {
   var pre = document.getElementById('console');
   var text = document.createTextNode(str + '\n');
   pre.appendChild(text);
 }
 
 function didOptIn(element)
 {
   // Force a synchronous style recalc and layout.
+  document.body.style.overflow = 'scroll';
+  document.body.offsetTop;
+  document.body.style.overflow = '';
   document.body.offsetTop;
 
   var nonFastScrollableRects = window.internals.nonFastScrollableRects(document);
   var elementBoundingBox = window.internals.boundingBox(element);
 
   for (var i = 0; i < nonFastScrollableRects.length; ++i) {
     var rect = nonFastScrollableRects[i];
 
     if (rect.top === elementBoundingBox.top && rect.left === elementBoundingBox.left &&
         rect.bottom === elementBoundingBox.bottom && rect.right === elementBoundingBox.right)
       return false;
   }
 
   return true;
 }
 
 function getStackingOrder(element)
 {
   var divElements = [];
   // Force a synchronous style recalc and layout.
+  document.body.style.overflow = 'scroll';
+  document.body.offsetTop;
+  document.body.style.overflow = '';
   document.body.offsetTop;
 
   var stackingOrder = window.internals.nodesFromRect(document, 100, 75, 200, 200, 200, 200, false, false, false);
 
   for (var i = 0; i < stackingOrder.length; ++i)
     if (stackingOrder[i].nodeName === "DIV")
       divElements.push(stackingOrder[i]);
 
   return divElements;
 }
@@ skipping 137 matching lines @@
       containerIndices.splice(containerSubindex, 1);
       var firstChildIndex = containerIndices[0];
       var secondChildIndex = containerIndices[1];
 
       permuteWithFixedOrdering(testPermutation, siblingIndex, containerIndex, firstChildIndex, secondChildIndex, 1);
       permuteWithFixedOrdering(testPermutation, siblingIndex, containerIndex, firstChildIndex, secondChildIndex, 2);
       permuteWithFixedOrdering(testPermutation, siblingIndex, containerIndex, firstChildIndex, secondChildIndex, 3);
     }
   }
 } // function doTest