Move traverseLiveNodeListForwardToOffset() to LiveNodeList class

Move traverseLiveNodeListForwardToOffset() to LiveNodeList class and rename to
traverseForwardToOffset() for consistency with
HTMLCollection::traverseForwardToOffset(). Also merge
traverseChildNodeListForwardToOffset() into
LiveNodeList::traverseForwardToOffset() as ChildNodeList is a LiveNodeList and
there is no reason to treat it differently from the other LiveNodeList
subclasses.

Finally, rename isNodeList(CollectionType) to
isLiveNodeListType(CollectionType) as it only applies to LiveNodeLists and not
other types of NodeLists (e.g. StaticNodeList). This check is always used before
casting to a LiveNodeList.

R=abarth, esprehn, eseidel

Review URL: https://codereview.chromium.org/120593002

git-svn-id: svn://svn.chromium.org/blink/trunk@164468 bbb929c8-8fbe-4397-9dbb-9b2b20218538