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

Source/core/testing/Internals.cpp

Index: Source/core/testing/Internals.cpp
diff --git a/Source/core/testing/Internals.cpp b/Source/core/testing/Internals.cpp
index 843d1694e0cccef4127e5b35a1b1044062d66315..f6e474b4f0ee989aee2e2054961a3b0b8c4c5bc1 100644
--- a/Source/core/testing/Internals.cpp
+++ b/Source/core/testing/Internals.cpp
@@ -1531,6 +1531,30 @@ String Internals::layerTreeAsText(Document* document, unsigned flags, ExceptionC
     return document->frame()->layerTreeAsText(layerTreeFlags);
 }
 
+void Internals::setNeedsCompositedScrolling(Element* element, unsigned needsCompositedScrolling, ExceptionCode& ec)
+{
+    if (!element) {
+        ec = INVALID_ACCESS_ERR;
+        return;
+    }
+
+    element->document()->updateLayout();
+
+    RenderObject* renderer = element->renderer();
+    if (!renderer || !renderer->isBox()) {
+        ec = INVALID_ACCESS_ERR;
+        return;
+    }
+
+    RenderLayer* layer = toRenderBox(renderer)->layer();
+    if (!layer) {
+        ec = INVALID_ACCESS_ERR;
+        return;
+    }
+
+    layer->setForceNeedsCompositedScrolling(static_cast<RenderLayer::ForceNeedsCompositedScrollingMode>(needsCompositedScrolling));
+}
+
 String Internals::repaintRectsAsText(Document* document, ExceptionCode& ec) const
 {
     if (!document || !document->frame()) {