Don't include headers from the layout API from other headers needlessly.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 30 matching lines @@
 #include "core/layout/LayoutGrid.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutListMarker.h"
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/LayoutMultiColumnSpannerPlaceholder.h"
 #include "core/layout/LayoutPart.h"
 #include "core/layout/LayoutReplica.h"
 #include "core/layout/LayoutTableCell.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/api/LayoutAPIShim.h"
+#include "core/layout/api/LayoutPartItem.h"
+#include "core/layout/api/LineLayoutBlockFlow.h"
 #include "core/layout/api/LineLayoutBox.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/layout/shapes/ShapeOutsideInfo.h"
 #include "core/page/AutoscrollController.h"
 #include "core/page/Page.h"
 #include "core/page/scrolling/SnapCoordinator.h"
 #include "core/paint/BackgroundImageGeometry.h"
 #include "core/paint/BoxPaintInvalidator.h"
 #include "core/paint/BoxPainter.h"
 #include "core/paint/PaintLayer.h"
@@ skipping 4782 matching lines @@
         m_rareData->m_snapAreas->remove(&snapArea);
     }
 }
 
 SnapAreaSet* LayoutBox::snapAreas() const
 {
     return m_rareData ? m_rareData->m_snapAreas.get() : nullptr;
 }
 
 } // namespace blink