Use tighter typing in table rendering code

Source/core/rendering/RenderTable.cpp

Index: Source/core/rendering/RenderTable.cpp
diff --git a/Source/core/rendering/RenderTable.cpp b/Source/core/rendering/RenderTable.cpp
index 8980b17d8798b7e85afba7bb7c58a020a9c1e638..ed3892362d30ddeb5e3e9d87e7bcf9755b5f2edc 100644
--- a/Source/core/rendering/RenderTable.cpp
+++ b/Source/core/rendering/RenderTable.cpp
@@ -4,7 +4,7 @@
  *           (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
@@ -602,14 +602,10 @@ void RenderTable::recalcCollapsedBorders()
     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);
             }
         }
     }