Don't remove empty display item pairs that draw content

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 "platform/graphics/paint/DisplayItemClient.h"
 #include "wtf/Assertions.h"
@@ skipping 216 matching lines @@
     DEFINE_CONVERSION_METHODS(SubtreeCached, subtreeCached, EndSubtree, endSubtree)
     DEFINE_CONVERSION_METHODS(BeginSubtree, beginSubtree, EndSubtree, endSubtree)
 
     virtual bool isBegin() const { return false; }
     virtual bool isEnd() const { return false; }
 
 #if ENABLE(ASSERT)
     virtual bool isEndAndPairedWith(const DisplayItem& other) const { return false; }
 #endif
 
+    virtual bool drawsContent() const { return false; }
+
 #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 void dumpPropertiesAsDebugString(WTF::StringBuilder&) const;
 #endif
 
@@ skipping 35 matching lines @@
     virtual bool isEndAndPairedWith(const DisplayItem& other) const override = 0;
 #endif
 
 private:
     virtual bool isEnd() const override final { return true; }
 };
 
 } // namespace blink
 
 #endif // DisplayItem_h