Avoid empty display item pairs

Source/platform/graphics/paint/DisplayItemList.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/DisplayItemList.h"
 
 #include "platform/NotImplemented.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #ifndef NDEBUG
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/text/StringBuilder.h"
 #include <stdio.h>
 #endif
 
 namespace blink {
 
 const PaintList& DisplayItemList::paintList()
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintEnabled());
     ASSERT(m_newPaints.isEmpty());
     return m_paintList;
 }
 
 void DisplayItemList::add(WTF::PassOwnPtr<DisplayItem> displayItem)
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintEnabled());
+
+    if (displayItem->isEnd()) {
+        ASSERT(!m_newPaints.isEmpty());
+        if (m_newPaints.last()->isBegin()) {
+            ASSERT(displayItem->isEndAndPairedWith(*m_newPaints.last()));
+            // Remove empty pairs.
+            m_newPaints.removeLast();
+            return;
+        }
+    }
     m_newPaints.append(displayItem);
 }
 
 void DisplayItemList::invalidate(DisplayItemClient client)
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintEnabled());
     m_cachedClients.remove(client);
 }
 
 void DisplayItemList::invalidateAll()
@@ skipping 118 matching lines @@
 #endif
 
 void DisplayItemList::replay(GraphicsContext* context)
 {
     updatePaintList();
     for (auto& displayItem : m_paintList)
         displayItem->replay(context);
 }
 
 } // namespace blink