Move traverseLiveNodeListFirstElement() to LiveNodeList class

Move traverseLiveNodeListFirstElement() to LiveNodeList class and rename to
traverseToFirstElement() for consistency with
HTMLCollection::traverseToFirstElement(). Also handle ChildNodeList in
LiveNodeList::traverseToFirstElement() as ChildNodeList is a LiveNodeList and
there is no reason to treat it differently from the other LiveNodeList
subclasses.

R=eseidel

Review URL: https://codereview.chromium.org/108343008

git-svn-id: svn://svn.chromium.org/blink/trunk@164802 bbb929c8-8fbe-4397-9dbb-9b2b20218538