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 6ee18f61e55afe37d3068e3a3602f06574072e3f..27c1393738151cdcab69fdd49717704d769b803b 100644
--- a/Source/core/testing/Internals.cpp
+++ b/Source/core/testing/Internals.cpp
@@ -1858,6 +1858,12 @@ String Internals::mainThreadScrollingReasons(Document* document, ExceptionState&
         return String();
     }
 
+    // Force a re-layout and a compositing update.
+    document->updateLayout();
+    RenderView* view = document->renderView();
+    if (view->compositor())
+        view->compositor()->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
+
     Page* page = document->page();
     if (!page)
         return String();
@@ -2291,4 +2297,18 @@ 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);
+}
+
 }