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

Source/core/rendering/RenderBoxModelObject.cpp

Index: Source/core/rendering/RenderBoxModelObject.cpp
diff --git a/Source/core/rendering/RenderBoxModelObject.cpp b/Source/core/rendering/RenderBoxModelObject.cpp
index 05743f6f538712356fb96c67ee003de927682691..f4dacfa008004a50214b69139cf1b2e848fa3eaf 100644
--- a/Source/core/rendering/RenderBoxModelObject.cpp
+++ b/Source/core/rendering/RenderBoxModelObject.cpp
@@ -2643,7 +2643,7 @@ void RenderBoxModelObject::setFirstLetterRemainingText(RenderTextFragment* remai
 
 LayoutRect RenderBoxModelObject::localCaretRectForEmptyElement(LayoutUnit width, LayoutUnit textIndentOffset)
 {
-    ASSERT(!firstChild());
+    ASSERT(!slowFirstChild());
 
     // FIXME: This does not take into account either :first-line or :first-letter
     // However, as soon as some content is entered, the line boxes will be