Clean up the way layout tests force elements to opt in/out of composited scrolling.

LayoutTests/compositing/overflow/build-paint-order-lists.html

Index: LayoutTests/compositing/overflow/build-paint-order-lists.html
diff --git a/LayoutTests/compositing/overflow/build-paint-order-lists.html b/LayoutTests/compositing/overflow/build-paint-order-lists.html
index efe610f8bebf69d028c25cba50a6f78eaa298fe1..cffeb99e80c91993df1f7fe40c8d5e3cf268e46d 100644
--- a/LayoutTests/compositing/overflow/build-paint-order-lists.html
+++ b/LayoutTests/compositing/overflow/build-paint-order-lists.html
@@ -80,6 +80,9 @@
     }
 
     function testPermutation(count) {
+      if (!window.internals)
+        return;
+
       var container = document.getElementById('container');
       // Here we want to compare paint order lists before and after promotion
       // to the actual stacking order as determined by hit-testing. So we
@@ -93,18 +96,22 @@
       container.style.webkitTransform = 'translateZ(0px)';
       document.body.offsetTop;
 
-      window.internals.settings.setAcceleratedCompositingForOverflowScrollEnabled(false);
+      window.internals.setNeedsCompositedScrolling(container, 
+          window.internals.ForceCompositedScrollingOff);
       container.style.webkitTransform = '';
 
       var oldStackingOrder = getStackingOrder(container);
       var oldPaintOrder = getPaintOrder(container);
 
-      window.internals.settings.setAcceleratedCompositingForOverflowScrollEnabled(true);
+      window.internals.setNeedsCompositedScrolling(container, 

[Julien - ping for review, 2013/05/06 22:56:59]

AFAICT no one is calling setAcceleratedCompositingForOverflowScrollEnable to
ensure that the feature is enabled now.

[Ian Vollick, 2013/05/08 13:10:58]

I think that the machinery coincidentally gets kicked into motion with this
setNeedsCompositedScrolling call (the setting is mostly used to determine if we
should waste resources computing the opt-in state) but it's very brittle. If any
bit of new logic gets added that looks at the setting, this test would break.
I've updated the test to turn the setting on.

+          window.internals.ForceCompositedScrollingOn);
       container.style.webkitTransform = 'translateZ(0px)';
 
       var newStackingOrder = getStackingOrder(container);
       var newPaintOrder = getPaintOrder(container);
 
+      window.internals.setNeedsCompositedScrolling(container,
+          window.internals.DoNotForceCompositedScrolling);
       // The getPaintOrder() function should return a pair of paint orders.
       // One before promotion and one after. This pair of lists should remain
       // identical whether the element is actually currently promoted or not,