Add documentation to several LayoutObject functions

Source/core/layout/LayoutObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 202 matching lines @@
     case INLINE_BOX:
         return new LayoutDeprecatedFlexibleBox(*element);
     case FLEX:
     case INLINE_FLEX:
         return new LayoutFlexibleBox(element);
     case GRID:
     case INLINE_GRID:
         return new LayoutGrid(element);
     }
 
+    ASSERT_NOT_REACHED();
     return nullptr;
 }
 
 DEFINE_DEBUG_ONLY_GLOBAL(WTF::RefCountedLeakCounter, layoutObjectCounter, ("LayoutObject"));
 
 LayoutObject::LayoutObject(Node* node)
     : ImageResourceClient()
     , m_style(nullptr)
     , m_node(node)
     , m_parent(nullptr)
@@ skipping 3172 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         fprintf(stderr, "Cannot showLayoutTree. Root is (nil)\n");
     }
 }
 
 #endif