Get rid of unnecessary isChildNodeList() virtual call in clearChildNodeListCache()

Source/core/dom/NodeListsNodeData.h

Index: Source/core/dom/NodeListsNodeData.h
diff --git a/Source/core/dom/NodeListsNodeData.h b/Source/core/dom/NodeListsNodeData.h
index 8717d1ec0eca6af845f2d8e1477be06a566784d7..2dd3923dd77741c9f922a943fe56b1216f199f5c 100644
--- a/Source/core/dom/NodeListsNodeData.h
+++ b/Source/core/dom/NodeListsNodeData.h
@@ -39,8 +39,11 @@ class NodeListsNodeData FINAL : public NoBaseWillBeGarbageCollectedFinalized<Nod
 public:
     void clearChildNodeListCache()
     {
-        if (m_childNodeList && m_childNodeList->isChildNodeList())
+        if (m_childNodeList) {
+            // This can only be called from ContainerNode so casting to ChildNodeList

[adamk, 2014/07/24 22:06:20]

This seems a bit unsafe, since there's nothing (except maybe an ASSERT in
toChildNodeList) stopping this from getting called on a Node.

Is this a performance win?

[Inactive, 2014/07/25 00:09:50]

Yes, there is an assertion in the casting function but I understand your
concern. Before I give up on this, I'd like to make a counter proposal:
- Remove the NodeListsNodeData::clearChildNodeListCache() method
- Add a NodeList* NodeListsNodeData::childNodeList() getter instead
- From ContainerNode::invalidateNodeListCachesInAncestors(), call
toChildNodeList(nodeLists->childNodeList())->invalidateCache()

Since we are in a ContainerNode method, the container node would look safer.
Loosing the clearChildNodeListCache() method does not seem important as there is
only 1 caller.

Maybe even better, the getter could even look like:
ChildNodeList* childNodeList(ContainerNode&);
// similarly to ensureChildNodeList(ContainerNode&) below.

What do you think?
I have just run Dromaeo but it did not show any perf win. I introduced this
virtual function call a while back and was not entirely happy with it, which is
why I was revisiting it.

+            // is safe.
             toChildNodeList(m_childNodeList)->invalidateCache();
+        }
     }
 
     PassRefPtrWillBeRawPtr<ChildNodeList> ensureChildNodeList(ContainerNode& node)