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

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 b37e3240ecec3cc3a62e301ac700b0e4f4aeb067..5f224e89e15f0b7a42ef8aa5d0bc104bbf0829da 100644
--- a/Source/core/testing/Internals.cpp
+++ b/Source/core/testing/Internals.cpp
@@ -2291,4 +2291,18 @@ bool Internals::loseSharedGraphicsContext3D()
     return true;
 }
 
+void Internals::forceCompositingUpdate(Document* document, ExceptionState& es)
+{
+    if (!document || !document->renderView()) {
+        es.throwUninformativeAndGenericDOMException(InvalidAccessError);
+        return;
+    }
+
+    document->updateLayout();
+
+    RenderView* view = document->renderView();
+    if (view->compositor())
+        view->compositor()->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
+}
+
 }