Don't remove empty display item pairs that draw content

Source/platform/graphics/paint/DrawingDisplayItem.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "platform/graphics/paint/DrawingDisplayItem.h"
 
 #include "platform/graphics/GraphicsContext.h"
 #include "public/platform/WebDisplayItemList.h"
 
 namespace blink {
 
 void DrawingDisplayItem::replay(GraphicsContext* context)
 {
     context->drawPicture(m_picture.get());
 }
 
 void DrawingDisplayItem::appendToWebDisplayItemList(WebDisplayItemList* list) const
 {
     list->appendDrawingItem(m_picture.get());
 }
 
+bool DrawingDisplayItem::drawsContent() const
+{
+    return m_picture->approximateOpCount() > 0;
+}
+
 #ifndef NDEBUG
 void DrawingDisplayItem::dumpPropertiesAsDebugString(WTF::StringBuilder& stringBuilder) const
 {
     DisplayItem::dumpPropertiesAsDebugString(stringBuilder);
     stringBuilder.append(WTF::String::format(", rect: [%f,%f,%f,%f]",
         m_picture->cullRect().x(), m_picture->cullRect().y(),
         m_picture->cullRect().width(), m_picture->cullRect().height()));
 }
 #endif
 
 }