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 7d5730be9860fa072df0efedae6b4437e2f4fdfa..9835cee6474bd60253592735816cfb43d7f1a3e5 100644
--- a/Source/web/PageWidgetDelegate.cpp
+++ b/Source/web/PageWidgetDelegate.cpp
@@ -38,6 +38,8 @@
 #include "core/page/Frame.h"
 #include "core/page/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 +80,10 @@ 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();
+
+    // FIXME: is this a reasonable place for this?

[jamesr, 2013/10/08 17:51:16]

this is reasonable since we don't have a better entry point on WebWidget for
this.  We do call WebWidget::layout() from the chromium side sometimes when
we're not about to make a frame, but most of the time this is the right place to
do the update.

+    // Finally do all the deferred work for updateCompositingLayers() here.
+    view->renderView()->compositor()->updateCompositingLayers(CompositingUpdateOnBeginDrawingFrame);
 }
 
 void PageWidgetDelegate::paint(Page* page, PageOverlayList* overlays, WebCanvas* canvas, const WebRect& rect, CanvasBackground background)