LayoutFullScreen::m_placeholder is LayoutBlockFlow.

third_party/WebKit/Source/core/layout/LayoutFullScreen.h

 /*
  * Copyright (C) 2010 Apple 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:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 13 matching lines @@
 
 #ifndef LayoutFullScreen_h
 #define LayoutFullScreen_h
 
 #include "core/CoreExport.h"
 #include "core/layout/LayoutFlexibleBox.h"
 #include "core/style/StyleInheritedData.h"
 
 namespace blink {
 
+class LayoutBlockFlow;
+
 class CORE_EXPORT LayoutFullScreen final : public LayoutFlexibleBox {
 public:
     static LayoutFullScreen* createAnonymous(Document*);
 
     bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutFullScreen || LayoutFlexibleBox::isOfType(type); }
     const char* name() const override { return "LayoutFullScreen"; }
 
-    void setPlaceholder(LayoutBlock*);
-    LayoutBlock* placeholder() { return m_placeholder; }
+    void resetPlaceholder() { m_placeholder = nullptr; }
+    LayoutBlockFlow* placeholder() { return m_placeholder; }
     void createPlaceholder(PassRefPtr<ComputedStyle>, const LayoutRect& frameRect);
 
 
     static LayoutObject* wrapLayoutObject(LayoutObject*, LayoutObject*, Document*);
     void unwrapLayoutObject();
 
     void updateStyle();
 
     // Must call setStyleWithWritingModeOfParent() instead.
     void setStyle(PassRefPtr<ComputedStyle>) = delete;
 
 private:
     LayoutFullScreen();
     void willBeDestroyed() override;
 
 protected:
-    LayoutBlock* m_placeholder;
+    LayoutBlockFlow* m_placeholder;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutFullScreen, isLayoutFullScreen());
 
 } // namespace blink
 
 #endif