Emit dummy display item when recorded picture is empty

Source/platform/graphics/paint/DisplayItem.h

 // 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.
 
 #ifndef DisplayItem_h
 #define DisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Assertions.h"
+#include "wtf/PassOwnPtr.h"
 
 #ifndef NDEBUG
 #include "wtf/text/StringBuilder.h"
 #include "wtf/text/WTFString.h"
 #endif
 
 
 namespace blink {
 
 class GraphicsContext;
@@ skipping 47 matching lines @@
         ClipBoxCollapsedTableBorders,
         ClipBoxTextClip,
         ClipBoxClippingMask,
         BeginTransform,
         EndTransform,
         ScrollbarCorner,
         Scrollbar,
         Resizer
     };
 
+    // Create a dummy display item which just holds the id but has no display operation.
+    // It helps a CachedDisplayItem to match the corresponding original empty display item.
+    static PassOwnPtr<DisplayItem> create(DisplayItemClient client, Type type) { return adoptPtr(new DisplayItem(client, type)); }
+
     virtual ~DisplayItem() { }
 
-    virtual void replay(GraphicsContext*) = 0;
+    virtual void replay(GraphicsContext*) { }
 
     DisplayItemClient client() const { return m_id.client; }
     Type type() const { return m_id.type; }
     bool idsEqual(const DisplayItem& other) const { return m_id.client == other.m_id.client && m_id.type == other.m_id.type; }
 
-    virtual void appendToWebDisplayItemList(WebDisplayItemList*) const = 0;
+    virtual void appendToWebDisplayItemList(WebDisplayItemList*) const { }
 
 #ifndef NDEBUG
     static WTF::String typeAsDebugString(DisplayItem::Type);
 
     void setClientDebugString(const WTF::String& clientDebugString) { m_clientDebugString = clientDebugString; }
     const WTF::String& clientDebugString() const { return m_clientDebugString; }
 
     WTF::String asDebugString() const;
+    virtual const char* name() const { return "Dummy"; }
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const;
 #endif
 
     virtual bool isCached() const { return false; }
 
 protected:
     DisplayItem(DisplayItemClient client, Type type)
         : m_id(client, type)
     {
         ASSERT(client);
@@ skipping 12 matching lines @@
         const Type type;
     } m_id;
 #ifndef NDEBUG
     WTF::String m_clientDebugString;
 #endif
 };
 
 }
 
 #endif // DisplayItem_h