add 'slow' prefix to RenderObject's firstChild() / lastChild() methods

Source/core/editing/htmlediting.cpp

Index: Source/core/editing/htmlediting.cpp
diff --git a/Source/core/editing/htmlediting.cpp b/Source/core/editing/htmlediting.cpp
index 392a28a37901dfbe89623ccc02404cf726e624af..659fd8c184786c381082987f4cd86b2d14e33309 100644
--- a/Source/core/editing/htmlediting.cpp
+++ b/Source/core/editing/htmlediting.cpp
@@ -52,6 +52,7 @@
 #include "core/html/HTMLTableCellElement.h"
 #include "core/html/HTMLUListElement.h"
 #include "core/rendering/RenderObject.h"
+#include "core/rendering/RenderTableCell.h"
 #include "wtf/Assertions.h"
 #include "wtf/StdLibExtras.h"
 #include "wtf/text/StringBuilder.h"
@@ -770,7 +771,7 @@ bool isEmptyTableCell(const Node* node)
         return false;
 
     // Check that the table cell contains no child renderers except for perhaps a single <br>.
-    RenderObject* childRenderer = renderer->firstChild();
+    RenderObject* childRenderer = toRenderTableCell(renderer)->firstChild();
     if (!childRenderer)
         return true;
     if (!childRenderer->isBR())