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

Source/web/PageWidgetDelegate.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 20 matching lines @@
 #include "config.h"
 #include "PageWidgetDelegate.h"
 
 #include "PageOverlayList.h"
 #include "WebInputEvent.h"
 #include "WebInputEventConversion.h"
 #include "core/page/EventHandler.h"
 #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;
 
 namespace WebKit {
 
 static inline FrameView* mainFrameView(Page* page)
 {
     if (!page)
         return 0;
@@ skipping 20 matching lines @@
     // they need to be told that we are updating the screen. The problem is that
     // the native widgets need to recalculate their clip region and not overlap
     // any of our non-native widgets. To force the resizing, call
     // setFrameRect(). This will be a quick operation for most frames, but the
     // NativeWindowWidgets will update a proper clipping region.
     view->setFrameRect(view->frameRect());
 
     // 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)
 {
     if (rect.isEmpty())
         return;
     GraphicsContext gc(canvas);
     gc.setCertainlyOpaque(background == Opaque);
     gc.applyDeviceScaleFactor(page->deviceScaleFactor());
     gc.setUseHighResMarkers(page->deviceScaleFactor() > 1.5f);
@@ skipping 118 matching lines @@
 {
     return mainFrame.eventHandler()->handleWheelEvent(PlatformWheelEventBuilder(mainFrame.view(), event));
 }
 
 bool PageWidgetEventHandler::handleTouchEvent(Frame& mainFrame, const WebTouchEvent& event)
 {
     return mainFrame.eventHandler()->handleTouchEvent(PlatformTouchEventBuilder(mainFrame.view(), event));
 }
 
 }