Get rid of inefficient uses of childNodeCount()

Source/core/dom/Range.cpp

Index: Source/core/dom/Range.cpp
diff --git a/Source/core/dom/Range.cpp b/Source/core/dom/Range.cpp
index 9c9ac3466ca2d477fbb7498f479c7d8c5a8071f1..458e7924d7a51f5e07133c5435916d894e547802 100644
--- a/Source/core/dom/Range.cpp
+++ b/Source/core/dom/Range.cpp
@@ -657,9 +657,10 @@ static inline unsigned lengthOfContentsInNode(Node* node)
     case Node::ELEMENT_NODE:
     case Node::ATTRIBUTE_NODE:
     case Node::DOCUMENT_NODE:
-    case Node::DOCUMENT_TYPE_NODE:
     case Node::DOCUMENT_FRAGMENT_NODE:
-        return node->childNodeCount();
+        return toContainerNode(node)->childNodeCount();
+    case Node::DOCUMENT_TYPE_NODE:
+        return 0;
     }
     ASSERT_NOT_REACHED();
     return 0;