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

Source/web/PageWidgetDelegate.cpp

Index: Source/web/PageWidgetDelegate.cpp
diff --git a/Source/web/PageWidgetDelegate.cpp b/Source/web/PageWidgetDelegate.cpp
index a5e7765a7ca6aadd5acfa097367f7ecc424dc999..2e245e22ff455443b6ac341de1624ca5b23ee50d 100644
--- a/Source/web/PageWidgetDelegate.cpp
+++ b/Source/web/PageWidgetDelegate.cpp
@@ -38,6 +38,8 @@
 #include "core/frame/Frame.h"
 #include "core/frame/FrameView.h"
 #include "core/platform/graphics/GraphicsContext.h"
+#include "core/rendering/RenderLayerCompositor.h"
+#include "core/rendering/RenderView.h"
 #include "wtf/CurrentTime.h"
 
 using namespace WebCore;
@@ -78,6 +80,11 @@ void PageWidgetDelegate::layout(Page* page)
     // setFrameRect may have the side-effect of causing existing page layout to
     // be invalidated, so layout needs to be called last.
     view->updateLayoutAndStyleIfNeededRecursive();
+
+    // For now, as we know this is the point in code where the compositor has
+    // actually asked for Blink to update the composited layer tree. So finally
+    // do all the deferred work for updateCompositingLayers() here.
+    view->renderView()->compositor()->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
 }
 
 void PageWidgetDelegate::paint(Page* page, PageOverlayList* overlays, WebCanvas* canvas, const WebRect& rect, CanvasBackground background)