Defer the real work in updateCompositingLayers until it's really needed.

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);
@@ skipping 146 matching lines @@
       }
     }
   }
 }
 
 function testOptInPermutation(count, ordering, hasPositionedAncestor, containerIsPositioned)
 {
   if (!window.internals)
     return;
 
+  window.internals.forceCompositingUpdate(document);
+
   var container = document.getElementById('container');
   var containerOptedIn = didOptIn(container);
 
   window.internals.setNeedsCompositedScrolling(container,
       window.internals.COMPOSITED_SCROLLING_ALWAYS_OFF);
 
   var oldStackingOrder = getStackingOrder(container);
 
   window.internals.setNeedsCompositedScrolling(container,
       window.internals.COMPOSITED_SCROLLING_ALWAYS_ON);
@@ skipping 57 matching lines @@
 
     permuteWithFixedOrdering(testPermutation, siblingIndex, containerIndex, firstChildIndex, secondChildIndex, 1);
     permuteWithFixedOrdering(testPermutation, siblingIndex, containerIndex, firstChildIndex, secondChildIndex, 2);
     permuteWithFixedOrdering(testPermutation, siblingIndex, containerIndex, firstChildIndex, secondChildIndex, 3);
   }
 } // function doTest
 
 function runOptInPermutationSet(permutationSet) {
   runPermutationSet(testOptInPermutation, permutationSet);
 }