Use tighter typing in table rendering code

Source/core/rendering/RenderTable.cpp

 /*
  * Copyright (C) 1997 Martin Jones (mjones@kde.org)
  *           (C) 1997 Torben Weis (weis@kde.org)
  *           (C) 1998 Waldo Bastian (bastian@kde.org)
  *           (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
- * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
+ * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
@@ skipping 577 matching lines @@
 // Collect all the unique border values that we want to paint in a sorted list.
 void RenderTable::recalcCollapsedBorders()
 {
     if (m_collapsedBordersValid)
         return;
     m_collapsedBordersValid = true;
     m_collapsedBorders.clear();
     for (RenderObject* section = firstChild(); section; section = section->nextSibling()) {
         if (!section->isTableSection())
             continue;
-        for (RenderObject* row = toRenderTableSection(section)->firstChild(); row; row = row->nextSibling()) {
-            if (!row->isTableRow())
-                continue;
-            for (RenderObject* cell = toRenderTableRow(row)->firstChild(); cell; cell = cell->nextSibling()) {
-                if (!cell->isTableCell())
-                    continue;
-                ASSERT(toRenderTableCell(cell)->table() == this);
-                toRenderTableCell(cell)->collectBorderValues(m_collapsedBorders);
+        for (RenderTableRow* row = toRenderTableSection(section)->firstRow(); row; row = row->nextRow()) {
+            for (RenderTableCell* cell = row->firstCell(); cell; cell = cell->nextCell()) {
+                ASSERT(cell->table() == this);
+                cell->collectBorderValues(m_collapsedBorders);
             }
         }
     }
     RenderTableCell::sortBorderValues(m_collapsedBorders);
 }
 
 
 void RenderTable::addOverflowFromChildren()
 {
     // Add overflow from borders.
@@ skipping 834 matching lines @@
 const BorderValue& RenderTable::tableEndBorderAdjoiningCell(const RenderTableCell* cell) const
 {
     ASSERT(cell->isFirstOrLastCellInRow());
     if (hasSameDirectionAs(cell->row()))
         return style()->borderEnd();
 
     return style()->borderStart();
 }
 
 }